Whats a good size perch because I just started catching them in a local pond and it started out a couple that were maybe five inches long and I caught one that was probably seven or eight inches.  They aren't very fat though so whats a good length/ weight.

I caught two 12 inchers on the same trip, last year, off a deep diving Rapala.  I thought those were pretty good sized. 
 
I've caught a lot of smaller perch, too.  Those are usually far more numerous than big ones.

If I'm going to keep a few to eat, the smallest that go in the bucket are 8-9". Any smaller than that and it's hard for me to get two fillets off of them. The first fillet is easy to take off, but the second one can be a real challenge and I don't like wasting fish if I don't have to. As to your comment about the fish being skinny, this time of year they will be. Perch generally spawn just after ice-out to early May, depending on water temperature. If you had caught those fish in February or March, they would have been much heavier.

My personal best is a 12-3/4" pig that was caught through the ice this past winter on Lake Winnibigoshish in Minnesota. I didn't have a scale, so my best estimate of the weight was around 1-1/2 to 1-3/4 pounds. Had it been another 1/4" longer, it would have met the taxidermist instead of the oil. It seems like perch get larger on bigger bodies of water. Lakes such as Devils Lake ND, Leech, Mille Lacs and Winnibigoshish in Minnesota, Winnebago in Wisconsin, and the Great Lakes have larger fish in them, and those perch can be upwards of 16"+. Not sure why this is; maybe more food/acre or less competition.

12 inches is a good size Perch but they are getting harder to find these days.

Perch do tend to grow larger in the larger lakes. Proceeding on the assumption that this is a discussion of yellow perch, I would consider a 12" fish very respectable. I just found out that there have been only 5 yellow perch over 2# documented as caught in Lk. Michigan even though the perch fishery there has be rebounding in the past few years. A 2# yellow would probably be close to 17".

my best was a 15-incher up in Caseville a few years back. you'd probly want at least 8-9 inchers for good filleting and frying

On my first fishing trip of the year, in unknown waters, I caught out of a stream, this perch. I would consider it big! Only a 2 foot deep stream while night fishing. Amazing to me anyways! I was psyched!