So I own the Hi-Tech jacket. As a reference, I wear a 42L suit coat. I want the body fit from the Dainese 52 (EU size/42 US size), but the sleeves are too short. If you have long limbs or torso you may need to go up 1 size from your usual. I bought the 54 to get a good sleeve length when I am on the bike. I am 6' 160lbs but my inseam is only a 32 so I am all upper body length....oh well.
BTW, why do you need to order on line? Is there anywhere you can try on a Dainese jacket? The fit will be very very similar across all their line.
On a side note, I absolutely love this jacket. I wear it year round. Liner in for the cold days with a windproof LS shirt under it and am perfectly fine. In the summer I only have overheating issues when it is 100+ outside, when I will swap to my Air-Tex, but it lacks the armor so.... Liner out and vent open.
Hope that helps.
-Joel