I replaced all of the rods of my original printrbot with 50cm rods and replaced the T5 belts with longer GT2 belts and pulleys.
Plus some changed details.
New build platform is now over 30 x 30 cm, and still printing nice. I have not testet all the limits yet, must add some feet to the base or it will tip over with big prints.
Experiences so far: It's louder than before, can't leave it printing during the night or my neighbours will kill me... it's building up more vibrations, also sortof a y-wobble. Most of this is probably because of lm8uu bearings with missing balls but the whole construction needs to be stiffened. If I had more space I would build a massive wooden frame around it.
Replaced the Base with Japica 608 Base, runs much smoother now
Nice detail: with this size, a 1m belt loop fits perfectly on the X carriage without having to cut it.
The Y belt is now in the middle of the build platform like http://www.thingiverse.com/thing:28423
For less Z-wobble I used the base pieces with Z motor support from http://www.thingiverse.com/thing:28557. Not used a Z - top joiner thing yet but it works fine without.
After adding some feet it will be more stable, but smaller prints turned out good. Only headache causing are the lm8uu bearings, I lost some BBs when changing the smooth rods. I will definitely try out the Japica 608 bearing thing at some point.
Warning, Printrboard firmware has software endstops in it, I can't use the whole build surface until i manage to flash a firmware that has it fixed.
fixed the firmware, was a bit tricky because of dfu bootloader (REV.B Board), but could compile it with the printrboard - arduino environment that can be downloaded at printrbot.com support. Then uploaded it with Atmel FLIP.
It wouldn't work at first because i used the wrong sources. Be sure to use the official printrboard sources from https://github.com/PxT/Marlin/tree/master/Marlin
Added the Z-Rod double stabiliser http://www.thingiverse.com/thing:36022