5 gals is fine for PM batches, you can even do some AG batches with smaller grain bills with it. I used a 5 gal tun for quite awhile and was able to mash ~12 lbs of grain in there. I did plenty of ~5 gal batches, all grain, and if the grain bill was over ~11 lbs, I either scaled the batch down, supplemented with some DME (partial mash) and on a few occasions, split the mash into two vessels. Worked great. That said, in the future, you'll probably want a larger mash tun. I recently moved up to a ten gal cooler so I could handle most any grain bill for ~5 gal batches.