This question pops up off and on. It is truly a loaded question. There are so many variables to consider. The end user of your product is what you should keep in mind. I would invest in several different gut sets and start sticking them into your calls. Tune them up and see what does and doesn't work with your call dimensions. Once you find a gut that is close, start monkeying with the call dimensions. Especially the inside bore of your mouthpiece. This will effect your sound and playability the most. Exhaust length will play into this some, but not as much.
Another option is to get a set of guts and start making calls until something works with the gut. I have heard that there is no bad set of guts, only bad tune jobs. Not sure if this is true, because I have had guts that just wont play. However, it could have been my call dimensions causing the problem.
Here is a set of dimensions that are a good starting point for Wade's SR-1 gut. This gut is probably one of the easiest for beginners to play, given a good call dimension. Also, with some tweaking it can be a really nice gut.
Barrel length approx. 3" and exhaust approx. 3" (w/ about 3/4" of tennon for O rings)
Bore your exhaust to 5/8". you will find that the SR-1 is slightly larger than this, but some sand paper will make the gut fit.
Bore your barrel to 5/8" and then back bore that with 7/8" for approx. 1" then 3/4 for 1". This will give you a stepped inside bore dimension in the barrel. This is only a starting point. These lengths will need to be manipulated to get the sound and playability you desire, but it should get you close.
Hope this helps.