Googling some benchmarks of the 2070 it can use high to ultra settings in games like Far Cry 5, Forza Horizon, Shadow of the Tomb Raider etc. and maintain over 60fps. So hopefully it should be quite a noticeable jump in performance. Some of that will be down to VRAM too, 8Gb is the minimum Iโd go for these days as games are getting chonky ao your 980 was probably having to do some VRAM management which definitely wouldnโt help game smoothness.
The motherboard is ATX form factor which means you canโt go too small but you can get some pretty small atx cases. Motherboad sizes are easy as the case will say what if they support it or not. Itโll just be some googling to make sure the 980 fits too.