The answer to your question depends on needle size, guage, number of strands and to be perfectly honest, length of time in the machine. Generally, I’ve found with a size 13 needle to end up with a 20-25" strap for a bag, I need to knit no less than 125 rows. When making a bag, I make it twice as long as I want the finished length. I won’t hurt to make it longer since a felted piece can be cut without worry of raveling. I’m almost positive that the larger the volume is of the item, the slower it will felt. So a guitar strap should felt faster. As for the width, I add at least a third more. So if you want it to finish at 2-1/4 inches you’d need to add 3/4". You really need to knit a swatch first and felt it to get it like you want. Just make notes on the size, number of sts and rows you used for your swatch and adjust according to the size after felting. Hope this helps… Mary
One other tip: The piece will felt more in length/rows than it will in width/sts…