What format? the larger the raster the larger the frame size is and thus will reduce the number of frames.
Also if it is a background then limit it to 24bit so that you don't waste cache on an alpha channel... also then it won't kick out another channel (1/3 and 2/4 are linked for Video+Alpha).
I would try to get a shorter BG though considering that every frame used for that is taking away form other cache frames. If you want long format you could also look at using the Clip player since it has no alpha channel and will playback h.264 files streamed off the USB .. you could make it like 10 minutes long :) and not have to worry.
------------------------------
Les O'Reilly
Director - Switcher Product Management
------------------------------