Re: Towables
What is your question? Too big for what?
I'm assuming you have plenty of power to pull a towable if you have a 17.5 foot boat. I have several sizes I pull. One is a 56" single rider tube, one is a 76" double rider tube, and one is a huge triangle shape two person raft looking thing that the riders sit in an upright position in. The lay down position on the tubes seem to be way more comfortable for the riders. I would consider two smaller tubes of the same size if you intend on more than three kids wanting to ride. You can get really creative with modern towables.

I can pull all of them behind my 16' aluminum fishing boat with a 75 Johnson. I can pull four people on two tubes, or five on three at the same time.
Dual tubes are the most fun you will ever have on the lake. I make all my ropes the same length so the towables will be side by side. When wake jumping they will bounce against each other like pinballs.
If I pull three tubes, the largest one is always in the center. It really gets fun when I get them behind my 7.4L Cobalt.
I use an electric inflator that came in one of my tube kits. It has a plug for a cig. outlet, along with an alligator clip adapter for power and works fast. You can also use it to deflate the tube if you want it vacuumed down to the smallest shape you can get.
I have several different size tow ropes, but the smallest one for the single tube will pull all you want it to. You won't break it during normal use.
This is the time of year they go on clearance pricing too. Have fun with the kids. :thumbsup