Does anyone know how these compare to amazon's slim fit chino shorts? those were a little loose on me. wonder if i should size down on these pants too

They are very comfortable around the waist, you should be ok if you go down a size.

Does theses come in regular or relaxed fit ?

Yes they are available in a relaxed fit and athletic fit, possibly even a straight fit/cut. In search box type: men's Amazon Essential relaxed cut pants. Or men's Amazon Essential, and change your results using the filters. Good luck and enjoy, I have 6 pair of slim fit and I get a size larger so they fit around my thighs better. I think you should try a pair using the Prime Wardrobe try before you buy option, you have nothing to loose. Remember, these are slim fit NOT Skinny fit, big difference ! Let me know if I can be further assistance please !

Do you measure from inseam or waist for length?

I just went by what I wear in any other pair of jeans. If anything, I would have gone up a size in the waist because of the slim fit. I couldn't tell you how to measure. Mine fit good though. I'm a big guy. Normally I wear 42w 32 inseam. I could use a little more room with these but they're still comfortable.