1995 Mustang factory Radio  dimensions?

Any single din should work just get the right wiring harness. I recommend getting a tablet single din 9" or 10.1" tablet single din head unit

Make your mustang modern like a tesla

Also make sure your head unit is totally dead. Check your fusebox and make sure you didnt blow your radio fuse.

Every respectable online retailer has a "what fits my car" feature so you can be sure to choose from HU's that fit.  The fact of the matter is just about everything will fit with the right dash kit and wiring harness.

Crutchfield.com is probably the best in terms of customer support, and they'll include the hardware you need for free (if you spend enough on the HU) or at a pretty good discount.

There are only two sizes of radios - single din and double din.  All of the pics I see of '95 Mustangs have a single-din, but there appears to be room below it if you wanted to install a double-din (although some modification would probably be necessary).

Make sure to get a wiring harness kit so that you can wire up the new radio  on a bench and then simply plug it in.

You can install any single DIN radio.  You just need an adapter if you have a double DIN stock unit.

Double DIN.  

You will need a kit if you want to use a single DIN head unit.  All car stereo shops, online shops and even Best Buy have sizing guides.