How can I make yarn from an old t-shirt and does the material matter? So my wife tells me that I have too many t-shirts (she is not wrong) as I don't wear them all fast enough before they get washed. Since I wear my favorites more often some just never see action. 

Naturally I will give some away but I have been fascinated with the idea of making my own crafts from that material e.g. yarn. I suppose I would just need scissors to cut it up but what can I do to make the longest strands possible?

What is a good way to determine if the cuts are too thin or not? I ask because the thinner I make it the longer strands I will get but the trade off might be strength. Too wide and it wouldn't make for good yarn. 

Does the thickness of the shirt matter? Not all shirts are made from the exact same material thickness. Is this limited to just jersey knit fabrics (That is typically what you see pictured with t-shirt yarn)?
It depends on what you're making with it.  When you're dealing with seamless T-shirts, I'd start at the bottom.  Cut the hem off, then go around and around cutting about an inch (2.5cm) in one continuous strip. You can put a little mark on your finger to indicate the measurement so you don't have to draw all those lines, and it won't need to be absolutely exact.  Don't bother with the neck and sleeves.  If you want some uniformity, you'll need to play with different widths.  You'll need narrower strips for heavier fabric.  Maybe make some test strips with the sleeves and make some samples before you start into the body of the shirts. 

The fabric is going to curl onto itself to form a strand instead of staying a flat strip if you stretch it at all.  If it's thin it will probably just do that anyway.  And that might be a good thing.