how do you know what size arrow or biscuit to get for your set up i want to shoot carbons

For carbons, get the carbon biscuit.

what about your shaft size of the arrow

I shoot the aluminum WB insert for everything. Works fine.

I saw your post elsewhere on choosing what size carbons you need. Each arrow manufacturer has the specs on their website. Just match your poundage, cam type, and draw length to their chart and you will be very close.

The very skinny outsert carbons or the new AXIS arrows may pose a problem with sliding through the bristles.
But for the most part the carbon biscuit will work for almost all of the ics carbons. The aluminum biscuit will work with them also.

A big fat aluminum arrow however will not work with the carbon biscuit.

There is a carbon biscuit with a .26" hole for the original outsert carbon arrows, and an ICS carbon biscuit with a .32" hole for either ICS carbon arrows or 20XX sized aluminium arrows. The aluminium arrow size biscuit has a .36" hole and will take up to a 23XX size aluminium arrow and still have a somewhat loose fit which is the reccomended fit for the arrow. It is easier to tune the rest and the bow with a looser fit between the biscuit and the arrow. The closer the tolerance between the arrow and the rest the more perfect the position of the rest has to be, and the more critical the tiller of the bow in order to get good results with this rest. Good luck hunting!

well you could go to your local sporting goods store they could help you out