How much do lithium polymer batteries expand in volume?
This paper measured a cell, they reported a max of 0.5% expansion over a charge cycle:


Source: Expansion of Lithium Ion Pouch Cell Batteries:
Observations from Neutron Imaging Figure 7  

Over the lifetime of the battery, it swelled more than 1.5%


Source: Expansion of Lithium Ion Pouch Cell Batteries:
Observations from Neutron Imaging Figure 9

One could use these numbers for a baseline, but it's not that hard to make these measurements to a reasonable degree. Since batteries are made with different anode\cathode and electrolyte combinations that vary from manufacturer to manufacturer, it would be wise to consult the manufacturer on swelling or measure it.

If you really want to find out how much your battery is swelling, get a micrometer and measure it discharged, then measure it fully charged and see what the difference is. Measure the cell under maximum discharge, because the cells swell more with thermal expansion. Give your self additional margin to account for differences in batteries and manufacturing tolerances. 

Cells also bulge more in the middle if they are heated then the outside. So make sure you measure the middle of the cell.


Source: https://www.researchgate.net/publication/283720424_A_novel_thermal_swelling_model_for_a_rechargeable_lithium-ion_battery_cell