Is a 27 inch monitor too big next to a 14 inch MacBook Pro?

No, it is the most common pairing. The monitor's picture is about 23.5 by 13.2 inches against the MacBook's 11.9 by 7.7, so it is roughly twice as wide. Sit about an arm's length away and it fills your view without turning your head.

How much desk do I need for a dual monitor setup?

Add the widths of the screens plus their bezels and a small gap. A 14 inch MacBook Pro beside a 27 inch monitor needs about 38 inches (96 cm), and two 27 inch monitors about 49.5 inches (126 cm), before you count the stands.

Why is the size on the box different from the drawing?

The number on the box is the diagonal, rounded. The drawing uses the visible picture: a 14 inch MacBook Pro is 14.2 inches, an iMac 24 inch is 23.5 inches and a Pro Display XDR is 31.6 inches.

What does looks like mean for Apple screens?

It is the size macOS lays out the desktop at by default. Apple screens draw each point with several pixels, so text stays sharp while menus and windows are a comfortable size. You can choose more space or larger text in System Settings > Displays.

Is anything I enter saved or sent?

No. The drawing is worked out in your browser and forgets your screens when you leave the page.

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Monitor setup visualiser: see your screens to scale

Pick up to three screens below and they are drawn side by side at their real physical size, with the width, height and pixel density of each and the total desk width. It starts with a 14 inch MacBook Pro next to a 27 inch 4K monitor: the monitor is about twice as wide and 70% taller. Swap in other sizes for a quick monitor size comparison before you buy.

Side by side: 37.8 in (96.1 cm) of desk, counting thin bezels and a 1 in gap between screens. Drawn to scale by the visible screen area.

Screen 1

Model

Width: 11.9 in · 30.2 cm
Height: 7.7 in · 19.6 cm
Pixels: 3024 × 1964
Density: 254 PPI
Looks like: 1512 × 982

Screen 2

Model

Width: 23.5 in · 59.8 cm
Height: 13.2 in · 33.6 cm
Pixels: 3840 × 2160
Density: 163 PPI

A typical model of this size.

How to read the drawing

Every screen is drawn at the size of its visible picture, so a 27 inch monitor is exactly as much bigger than a MacBook as it will look on your desk. The ruler under the drawing marks every inch.
Screens sit with their bottom edges in line. Real stands lift monitors higher than a laptop, so on the desk the tops end up closer than they look here.
Bezels are drawn the same thin width on every screen and the gap between screens is 1 inch, so the total desk width is a close estimate.
Pixel density (PPI) says how sharp text looks. Looks like is the size macOS draws things at by default on Apple screens, which is why a MacBook Pro 14 inch with 3024 by 1964 pixels shows a desktop like 1512 by 982.
Apple sizes come from Apple's specs. The iMac 24 inch is 23.5 inches across its visible picture and the Pro Display XDR 31.6 inches, and the drawing uses those.

What size works next to a MacBook

A 27 inch monitor is the usual partner for a 13 to 16 inch MacBook. It is around twice the laptop's width, so a window you drag across roughly doubles in room without forcing you to turn your head.

macOS draws its interface at 1x or 2x, so it looks crispest near 110 or 220 PPI. A 27 inch 5K screen such as the Studio Display (218 PPI) matches the MacBook. A 27 inch 4K (163 PPI) looks good at a scaled setting. A 27 inch 1440p monitor (109 PPI) shows visibly softer text than the laptop beside it.

A 32 inch 4K (138 PPI) or a 34 inch ultrawide (about 110 PPI and 31 inches wide) gives more room for side by side windows, at the cost of sharpness and a lot more desk.

What happens to your windows when you dock

When a monitor connects, macOS leaves the windows that are already open where they are, so most people drag the same apps across every morning. macOS can bring windows back to a display it has seen before, but not always, and after sleep they often end up piled on the MacBook screen.

With DisplayShift, a panel drops out of the notch when a display connects, listing every open window, and you tick which ones move. It remembers a layout for each desk, puts windows back after sleep and tiles up to four apps side by side. It costs $6.99 once.

Going further: How to set up two external monitors on a MacBook.

Questions

Is a 27 inch monitor too big next to a 14 inch MacBook Pro?: No, it is the most common pairing. The monitor's picture is about 23.5 by 13.2 inches against the MacBook's 11.9 by 7.7, so it is roughly twice as wide. Sit about an arm's length away and it fills your view without turning your head.
How much desk do I need for a dual monitor setup?: Add the widths of the screens plus their bezels and a small gap. A 14 inch MacBook Pro beside a 27 inch monitor needs about 38 inches (96 cm), and two 27 inch monitors about 49.5 inches (126 cm), before you count the stands.
Why is the size on the box different from the drawing?: The number on the box is the diagonal, rounded. The drawing uses the visible picture: a 14 inch MacBook Pro is 14.2 inches, an iMac 24 inch is 23.5 inches and a Pro Display XDR is 31.6 inches.
What does looks like mean for Apple screens?: It is the size macOS lays out the desktop at by default. Apple screens draw each point with several pixels, so text stays sharp while menus and windows are a comfortable size. You can choose more space or larger text in System Settings > Displays.
Is anything I enter saved or sent?: No. The drawing is worked out in your browser and forgets your screens when you leave the page.