I plan on using black pipe and was thinking of using 3/4". As well i plan on doing all the work except the final conections (i will hire someone)
Do not even think about using black pipe if you are going to bury it underground. It will not pass any code, and WILL rust out in a very short time.
1) can i oversize my line without (3/4 instead of 1/2) any problems as im not sure what size BTUs the furnice is.
3/4" would not be oversizing it. In fact it might be too small depending on the distance from the meter to the furnade and the furnace's BTUs.
3) i have a 3/4" line going into the house (main line) and i have a gas furnice, dryer and in the future a gas stove as well as a furnice in the garage is this line even big enough??
Probably not, but it depends on the length of the pipe and the BTUs being used.