Awww, super cute! Unfortunately, a ragdoll mix isn't a ragdoll at all, and it's highly unlikely that the cat is "half-ragdoll" to being with. I know with Savannahs it's different, that you can kinda figure out what they are by what they look like; but a ragdoll is a specifically-bred cat, and several other breeds (especially the completely unrelated Birman) can often be confused with them. Due to the ragdoll breeding program (which is relatively new years-wise) it's highly unlikely that a true ragdoll would be wandering outside randomly breeding with stray cats. Also ragdolls have a super-laid back personality...so much so, that they really can't be let outside on their own, as they have very few faculties for defense (think the complete opposite of Savannah). Absent being able to trace a microchip or paperwork, it's more likely that your brother's kitty is half Siamese and half long-hair cat, or maybe half maine coon?, which would give it it's pointed look and long hair.
At any rate, he is really cool, and most people that seek out the ragdoll breed are seeking out certain characteristics, such as the laid-back personality, floppiness, big bone structure, pointed or bicolor patterns, etc. so if you got those characteristics in a non-ragdoll cat then consider yourself lucky! Once you have adopted a pet cat, unless you were planning on showing or breeding, it's pedigree really doesn't make much difference...other than, like you said, to know what to expect.
As far as the size of a typical Ragdoll, some can be as big as 30 lbs or more...my Dante is rather small for his breed, at 12 lbs, so it can vary quite a bit. Ragdolls have a fat pad on their bellies making them look like they're actually fatter than they are.
Just for kicks, here is a picture of a Birman flame point:
and a Ragdoll flame point: