Tank Lid ventilation

[Cindy Slatcher]

I bought a piece of plexi glass for the top of the new tank I had given to me. I need 
to drill holes in it for ventilation any suggestions on hole size and how many? The 
piece of plexi is 1x3ft. I plan on putting Clifford and his 4 boys in this tank when 
they are old enough, I'll put Spikey and her daughter in a 20 gallon. Any suggestions?

Thanks!