I had a lab that would not use his 8'X13' dog run as a bathroom, he would hold it for eternity until I would let him out. Sometimes I think he pissed for 15 minutes, if it was a overnight late morning deal prior to letting him out.
We had a small dog that took awhile to learn how to potty train. What worked well is we put her into a kennel at all times unless she went potty outside. Then we let her run around the house for an hour or two before we took her outside to go potty again. If she went potty we let her run around. If she didn't we put her in a kennel for awhile and tried again. It didn't take long for her to learn.