A girl buys a size that fits her, but is too long for her. So she says:
I guess I'll have to cut it down to my "length/size". (It fits her perfectly but is too long)
So what should be used "length/size"?
She would say "I'll have to cut it to the right length."
Size in clothing would be more likely to indicate that it's too big in terms of width.
Specifically in this case, she's more likely to say "It fits nicely, but I'll have to hem it" which means to cut or turn under the excess material and sew a new bottom edge.
If the length was right but the dress was too big or wide on her, she might say "The length is good, but I'll have to take it in," which means to remove excess fabric width-wise.