u still will work your muscles, doing any kind of resitance training just.
make sure u know what :
Heavy(STRENGHT) 1-5 reps 2-3 min rest
hypertrophy (muscle building) 4-10 reps 1-2 min rest
muscular endurance 10-20 reps 30seconds - 1 min rest

oh man, thanks, so it s possible to decrease muscle size if i lift too light too long though?

no it's more like active recovery, you will never experience any kinds of termendous gains doing light weight, but you will not lose muscle either.

You can definitely stay fit using light weight but if you want to get big you will have to progressively use heavier weights. You shouldn't have to worry about decreasing muscle mass from lifting to light for to long as long as you have enough recovery time as Beast said.

try working up to one arm pull-ups on a tree out back or the beam in your base ment that supports the house , and try one armed push ups . once exhuasted on these switch to two hand versions . you can even pre-exhaust your delts and arms once in a while with these super highreps before pullups/pushups .
if your to heavy LIKE I AM to do these things just do both hands at same time with all diferent grip widths etc . !