I’m 6-1 and will be at or under 160 for in season weight. I feel like I’m between a M and L in castelli. M is very compressive but short as crap. L was great initially, but material has lost a little stretch over time. Still, I’d go large though (mediums work great on the trainer though). Both are Endurance model with the progetto x2 pad.
For all other shorts I’ve worn I’m solidly M - giordana, LG (though the power band model was tight at the gripper), craft, mt borah, champ Sys, sugoi, PI with the 4d chamois, and all are long enough for me.
Eta: if the manufacturer published inseam, 9 inches is usually good for my build. Though I am accustomed to shorts being hitting a little higher on me than shorter riders or riders my height with a l/xl. Castelli M is a definite outlier in how short it is though
Another eta: my team is wearing MOA this year. A guy who is 6-5 and 170 felt most comfortable in a small. He has very skinny legs, but they were long enough for him. My legs are thicker and the gripper band would be too tight on me in a small.