Depending on your use, you use separation and control joints.
There are a lot of guides available that cover this subject, but it can vary quite a bit depending on use, thickness and available friction surface. Most guides will call for roughly rectangular pieces. Slabs tend towards 1:2 to 1:4 ratios on width to length.
Slabs are based upon friction stress breaking up the slab. Walls tend towards the ability to “tilt-up” the section without cracking.
Of course, I’m making assumptions of what you are making out of concrete.
As a last note, most texts will state that concrete will shrink by about 4%. I will tell you that that is a worst case scenario, and that most cases that shrinkage on site tended more towards 1%-2%.
MKDesign 