you have a 25 cubic foot area. in theory u would want at least a 5-10 cfm fan at least sucking air out all the time. but since your having heat problems i would change the air in there every 30 seconds, that wont allow heat to build up from the lamp. that might be a little to much tho and might create a windtunnel lol. i would look for a 50-75 cfm fan that u can rig up to exhaust your box. this should be your main priority right now, you cannot grow without proper ventilation. as for intakes, you always want your intake hole to be larger than your exhaust hole.