Did you adjust the firmware settings, does your printrbot "know" it has a bigger bed now?
Looks like this can be done by sending a M211 X/Y300 (X/Y Axis, 300mm*), followed by M500 (save) and M501 (readout, check if setting is saved)
I do not own a printrbot, but one of the threads I came across was this one:
https://www.thingiverse.com/groups/prin ... topic:4271
* M211 X300 for x axis _or_ M211 Y300 for the y axis