What gauge wire for long run from power supply to LED light strip?
Is 5050 the model number or the quantity of leds?

The cable you linked to looks very lightweight - you don't need a lot of copper to run leds directly off of low voltage DC but you do need to handle it a lot during installation.

I would get something heavier, just for the mechanical advantage. 16AWG stranded 300 volt should do nicely, the voltage rating is to get thicker insulation. You can get spools of it in many colours from any electronics supplier (not electrical, "electronics") in the local phone book. Also pick up an automatic wire stripper and a decent crimping tool while you are there - the common stamped-steel crimpers do NOT do a good job, and you don't want to have to get in behind the walls twice.