The only thing about scroll saw arm length is that longer arms vibrate more. I have a Craftsman 20" scroll saw that came with all the bells and whistles. I never use it because it rips out the back of wood no matter what blade I use.

I also have a 15" delta knock-off that I paid $99 for about 20 years ago. Using the same blades it gives great cuts so that's the saw I use. My Craftsman saw sits in a corner with a drop cloth over it.

A 14" to 16" model will do 99.9% of everything you need it to.