So by 10%, I assume if my model's wingspan is 10% or more than the size of the full scale it qualifies?
No... the wingspan determines the scale of the model, say 27%. Now once the scale is set, all other dimensions such as length, height, stab span, wing chord, etc must also be 27% of the full scale, +/-10% deviation. So, say the length should be 54" to be perfectly scale, the length can vary plus/minus 5.4" (10% of 54" )... so the length can be anywhere from 48.6" to 59.4" and still be "within 10% of scale".
That being said though, I've never seen that checked or enforced except at the TOC.