size

i weight about 65/70 kg (dunno how many lbs that is), i'm 1,77m and im riding with a 138. why is it better for me to get like a 134? would landing be softer and maybe even easier on a 139 or something???

if i'm correct, you're about 5'8" or so, and about 150-160 pounds. assuming this is true, I would suggest staying around a 135-138 area for your boards. I'm sure it would be possible to ride on a 134 and a 139, but there's two reasons why you shouldn't buy a 134.
1. if you weigh higher than around 150-160, a 134 board isn't big enough to support your weight.
2. in your profile it says that you're 13 years old. you are bound to grow and you will no doubtedly grow out of a 134 board. unless you want to buy a new board to fit you a year from now, i suggest you stick with a board around the size you have now.