As for size, no, 3/4 is a size most middle school kids use. Once in high school, most players are ready for 4/4 size instruments. A VERY VERY general rule of thumb for sizing would be to stretch as far as you can between pinkie and index finger and measure that.
If you can reach X” you can probably play X/X size:
4″ = 1/2 size
5″ = 3/4 size
6″ = 4/4 size
The cello itself should fit comfortably so the point of the lower bout rests at the side of your knee, the top of the cello’s main body rests sqarely in the middle of your chest (on the breastbone), and the pegs are just behind your left ear. If you have a cello that’s too small, it will be cramped to play.
Also, if you hold the cello like that, you should be able to cross your arms over the fingerboard and touch the opposite corner points. If you can’t reach them, the cello is probably too big for you.