Dell uses (largely) proprietary motherboards. I can't tell you how different yours is from a standard ATX motherboard.
The best way to find that out is to post this question on Dell's user forum.
Generally, not only is the board a different size (as you have seen), but the front panel connectors are often proprietary too. That means you might have to rewire or give up your front panel connectors. And the Dell psu you have may or may not work in a standard ATX motherboard.
If you *are* successful dropping ina new mobo, you face an OS problem. Booting up a new mobo with your old C drive is a dicey proposition. There are guides on the Internet that will show you how difficult it can be.
Most likely you do not have a Windows CD, but rather an image of the "C" drive as it was originally shipped to you. That limits certain other things you might have to do.
You *could* buy a new mobo, case, psu, and OS . . . drop in your existing cpu, drives etc if you like and do a fresh install. But that's basically a new system, so we come back to the question "Why do you want to upgrade your mobo?"