Calculate power requirement for 64 rgb led ws2812b
If all LEDs are in full bright white, assume 60mA per LED. 64 would mean a maximum current draw of 64*60mA=3840mA. This figured would not include the Arduino itself. 

a 500mAh battery could power this for no more than 7.8 minutes (500mAh / 3840mA = 0.17 hours = 7.8 minutes)

If your usage will not be full bright 100% if the time, then you could run longer. 

You didn’t say how long you want it to run. Perhaps start with the design goal before specifying parts. 

For a more realistic view of how much power it takes, measure with a multimeter the current draw from the battery or bench power supply to your boost converter while displaying a typical usage on the matrix. Use calculations as above to determine the expected battery lifespan.