Take your 5×5 to a real scanning photo place and have the scanning done properly (cause I am pretty darn sure with a photo that old you don’t have the original negative! [it’s probably crumbled and disintegrated anyway by now])
have them print it there if you can and their prices are good, otherwise ask if they can put the scan on a flash drive or something for you.
If you'd rather it be print online, MPix is okay, rather expensive, I have all of my prints done by these guys, good service, fast, and very cheap: http://www.fullsizeposters.com/
you just upload your picture and can set the dimensions from there.
That picture you uploaded though, there’s no point in trying to go into photoshop and blow it up.
Even if you could like rent a scanner right now you’d get a much better size than that.
Heck if I used my 8 year old printer/scanner combo on high settings not only would I land up with a 30 megapixel+ picture, it would look tons better than that 0.3 megapixel blown up…
and my scanner is a POS lol
Seriously, take it to a camera store and have them scan the original photo, that would make sure it’s as clear as possible, and if they can, print it there or elsewhere.
just get that 5×5 scanned somewhere professionally!