The way to decide what you need is as follows, comfort is everything,  Remember when we were kids and you were taught what bike was too big,  technically, with a bike the rule is you need to be able to sit on the sit with the sole of one foot completely on the ground and the knee partly bent, the other foot on the pedal

With guitars, as a general rule….
1. when you look into the palm of your relaxed hand, notice the distance between your fingers , then put that hand on the fretboard,  the frets should correspond to your fingers, you should be able to pick 4 frets sequentially  from 1st finger then second finger then 3rd, then pinky  on the one string,  if you find that your fingers are cramped on a 3/4 then you need a full size guitar.

2. Now.. to check Length,  put 1 of your fingers , any one, doesn’t matter, on the 1st Fret, right at the top of the neck,  and see how your wrist feels, Now, press down, 1 by 1 each string on the 1st fret,  Paying special attention to the thickest string,
do this on a full size guitar,   if you find it’s really difficult and your overstretching your fore arm muscle,, you definetely need a 3/4 size

3. repeat the exercise in step 1 , but do it on frets 1 then 2 then 3 then 4  on the thickest string
 so basically your playing the following notes
1 st Fret Bass E string = F Note
2nd Fret Bass E string = F Sharp Note
3rd Fret Bass E Sting = G Note
4th Fret Bass E string = G Sharp Note

See how you feel with this exercise,  You should feel slight tension in your arm, because your not used to it, but certainly not excessive tension, if that’s the case … get a smaller guitar

Hope this helps,

Hope you stick with it, Don’t take excuses from people Man,
i was told in the beginning, my fingers were too fat,…… What a load of B$%LSH#T
if i had listened to people talking rubbish i would have missed out on the intense pleasure that i get from Music….. Trust me,  Just keep playing,  it’s not easy,  But definitely reward… More than you now, know

Be Cool

when doing these exercises  remember that you will need to reach these positions with speed