What tie width will be best for you depends on your frame. If you're a smaller guy, you may want something like a 3". If you're a larger guy, 3.75". Or just buy something within that range and don't obsess- it doesn't matter that much anyway, as long as it's not a huge mismatch with your jacket lapel or frame.
As for the difference between silk, wool and cotton, they're made out of different things. Silk is typically smoother and more formal, and is an all season choice, cotton is lighter, wears a little cooler and has a more casual texture with less sheen. Wool is rougher, has no sheen, and is again a more casual alternative. Wears warmer, obviously.
I'm a "small guy" ( 5' 5", 130 pounds) but semi-big for my size (athletic). I'm not sure about my jacket lapel since I haven't received my MTM suit yet. I know the standard is 3.5" and I'm planning to buy some ties from TheTieBar to start off my collection. Do you think 3" or 3.5" would be more suitable? I have a 2.75" at its widest point and I wouldn't mind if it was a little bigger so I'm leaning towards 3". However, on TheTieBar, it lists 3" as skinny (though I don't think it really is).
And I guess I'll go for silk. Is that what people normally use?