Typically, the shorter the better. I feel like I get better penetration and less hangups with a bait that is 3 3/4 to 4 inches. If you have a mat that is ridiculously thick, it's better to have the shortest and most compact bait you can. The only exception would be a senko or similar bait.
"If people concentrated on the really important things in life, there'd be a shortage of fishing poles." - Doug Larson