@aardvark Yeah that really depends on the number of games you have. So that is hard to nail down until you actually have everything scraped. Each video is a different size. For example an Atari 2600 games snap that is 30 seconds long will be much smaller than say a PSX games of 30 seconds. The videos that are scraped and converted for the pi are 30fps @ 320x240 4:3 aspect. Great for the pi and they are smaller size too! (the vidsnaps can be 640x480 @ 60fps 4:3 the pi can handle them..but the converted ones work much better!)
Honestly a one terabyte hard drive will get you most of everything the pi can handle. However a 128gb micro sd card is "perfect" and a 256gb should be more than enough..unless you want to run a LOT of psx games. (This does not include PC-ish games.) My system with games, videos and snaps fit on a 64gb card happily (and that includes daphne). Note though that i pulled out alot of arcade games (clones/mature/mahjong).