I have a normal 3DS (LoZ 25th anniversary version) & find the screen size to be absolutely fine.
When 3D is enabled, quite a few people have issues finding a perfect position to maximize the 3D itself, without hurting their own eyes.
I have not ever used, or owned a 3DS XL, so I cannot say anything positive or negative about it.
In regards to the classic however; if you find it on-sale, than picking it up is a fine idea.
It does its job, & I'd say it does it well.