A few ideas from experience.

Make it as large as the space can handle. It's easier to keep a bigger one watered and little boxes look scrappy. You want it to look generous and overflowing.

Remember it needs drainage holes and therefore a drip tray if there's danger of staining your wall/window sill or soaking passers-by.

Make sure it is well-secured to the house if the sill is narrow or you get high winds.

Use a liner so you can swap the displays without having to hang out of the window or carry a heavy damp box of earth round/through the house. Especially useful if they are on upstairs windows.

Choose a style/colour/material to match the house and your climate.

Avoid plastic if you can possibly afford to (except for the liner). It looks tacky, the colour will fade and it could well bend out of shape when full of compost.

Make it BIG. Big, big, big! You can support with multiple brackets beneath.