Welcome to the madness.
This is an off-the-cuff reply, as I have no idea what your in-dash stock nav looks like.
This forum is a definite DIY thing, so please honor yourself by taking the time to read a LOT of posts, and do a LOT of searches. Tedious tho it may be, reeeeeally explore the information presented here. That approach has served many of us well.
Here's a suggestion how to begin: get a ruler and measure your in-dash nav. Also, take some pics, and add them to yr profile. People can offer better suggestions if you provide detail...
A lot of dash moldings or bezels unclip or otherwise come off pretty easily, so investigate and pull yours off if poss; remove the nav if poss to get idea of what room you've got to work with. If you know what yer doin', ya kant hurt nuttin'.
There ARE smaller screens, but I'm growing old and blind, so I have a Lilliput 629 7" widescreen. The outside dims of case are 7-1/4"w x 5"h x 1-1/8"d.