The size limit for the LCD frame buffer is 1024x1024x4, which would be a 1k x 1k x 24bpp display. You can compute a frame buffers memory requirements by multiplying the X resolution by the Y resolution by the color depth in bytes. For example, a 320x240 display at 16bpp (2bytes per pixel) would need 320x240x2 = about 150kB. You can also use the bandwidth calculator to compute this value.
LPC247x bus bandwidth calculator:
LPC178x and LPC40xx bus bandwidth calculator:
LPC18xx and LPC43xx bus bandwidth calculator: