Well like he said size and weight depends. I would not get a 2-stroke your gonna wanna get a four stroke if your a beginner
If your 7-10 years old I'd recommend an 80
10-12 a 100
13-18 a 125
and if your experienced and know how to ride the 125 fine and you are ready to move up then get a 250 or you can up the horse power by getting a big bore kit, new exhaust, and much much more. by then you'll probably know what you wanna do
A good website to look for a dirt bike is Craigslist.org and if your gonna get a dirt bike make sure that the frame isn't cracked anywhere, it doesn't have engine problems, and the bike starts when you go and look at it