While not as spacious as Palazzo/Venetian, they do have nice rooms which comparable to Cosmo and the others. It may be the same size or even bigger.
The difference between Signature and the others is there's a stove and microwave which means you can reheat some leftovers or cook some snacks. It comes in handy.
The only negative about Signature is it's off strip. I mean, really off strip. The only way to get on strip is to take a long walk through a tunnel which leads to MGM. It's not that bad but MGM is on one end of the strip and when you want to hit up Encore, it's definitely taxi time. Also, there is no casino in here.
The parking is a little wonky too. You need to be a guest to park here and there's no self-parking, it's all valet.
Besides those little details, it's a nice place to stay. Quiet, but nice.