I usually find two common sizes of copper clad boards, which are 10*7cm and 5*7cm. It can be useful at times to split a 10*7 into two 5*7.
Cutting them with a handsaw or dremel is a pain and makes a lot of dust, to make it better  I've made this little jig that guides your box cutter to score both front and back of the PCB at the center, after a bunch of passes you can just split it by hand and it will break along the scored path.
It requires two 8cm sections of 6*6 U-shaped aluminum extrusions, two M3 heated inserts, and two M3*8mm screws. 
After scoring the edges might come up a bit, I suggest using a deburring tool. 

Note:works well for FR1 not so well for FR4