I need a 16" seat for a saddle but the one that I found is a 16.5". I still want growing room but not too big. Do you think the .5" will make huge a difference?

No. I think a size that is a bit bigger is better for someone who is still growing!!!...🐴❤️
16.5" is good. If it fits your horse, that is. You could even ride in a 17" saddle and you'd be fine... so don't worry about your size and take what your horse needs. If you're just worrying about your personal comfort I think the 16.5" will be perfect. :)
I would always recommend getting a saddle that is a 100% fit.
No it should be fine.
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