Snapsize

Snapsize is a window resizer for people who build websites, especially on big monitors where dragging the window never lands on the size you need.

Exact sizes, one click

• Phone, tablet and desktop presets up to 4K. Rename, reorder, add or delete any of them.
• Sizes the page area (what your CSS media queries see), not just the outer window. Toolbars and sidebars are measured and corrected for. Whole-window sizing is one click away.
• Rotate phones and tablets to landscape.
• Phone widths narrower than Firefox allows for a normal window open in a slim mini window instead.

Breakpoint aware

• Tailwind, Bootstrap and MUI breakpoints built in, plus your own.
• A -1px toggle jumps to one pixel below each breakpoint, where max-width bugs hide.
• The popup and the toolbar badge show the live width and which breakpoint range you're in, even while you drag the window by hand.

Sweep

• Step through every breakpoint or device on a timer, or slide smoothly from narrow to wide and watch where the layout jumps. Optionally save a screenshot at every step.

Compare

• Open the current page in up to four exactly-sized windows, tiled side by side. Reload them all with one click after a code change.

Screenshots

• Save a PNG at the current size, named after the site and size (example.com-768x1024.png). Visible area or full page, native sharpness or exact 1x pixels, in the folder you choose.

Everyday conveniences

• Keyboard shortcuts for four favourite presets, screenshot, restore and "this site's size".
• Restore puts the window back exactly how it was before you started, including maximised.
• Remember a working size per site, for example localhost:3000.
• Export and import presets as JSON to share with a team. Optional Firefox Sync.
• Light and dark themes. The popup never scrolls: it grows sideways to fit your presets.

What it can't do Snapsize resizes real windows. It does not emulate touch input, device pixel ratio or a mobile user agent. Use Firefox's Responsive Design Mode (Ctrl+Shift+M) for those.

Privacy Snapsize collects no data and makes no network requests. Everything is stored locally in your browser. If you turn on the optional sync setting, Firefox stores your presets in your own Firefox account.