I keep losing this information in the Next manual, so here it is for quick lookup.

Layer Name Resolution Cell size Cells Colours Colours/cell
L0 ULA 256 x 192 8 x 8 32 x 24 8 2
L1,0 LoRes 128 x 96 - - 256 -
L1,1 Standard Res 256 x 192 8 x 8 32 x 24 256 2
L1,2 Timex HiRes 512 x 192 - - 2 -
L1,3 Timex HiColour 256 x 192 8 x 1 32 x 192 256 2
L2 256 x 192 - - 256 -
L3,0 Text mode 320 x 256 8 x 8 40 x 32 256 2
L3,1 Text mode 640 x 256 8 x 8 80 x 32 256 2
L3,2 Graphics mode 320 x 256 8 x 8 40 x 32 256 16
L3,3 Graphics mode 640 x 256 8 x 8 80 x 32 256 16

In addition to these layers, there are also the sprites which don't work off a grid.

Layer 0

  • Layer 0 – Standard Spectrum (ULA) mode, 256w x 192h pixels, 8 colours total (2 in tensities), 32 x 24 cells, each capable of displaying 2 colours

Layer 1

  • Layer 1,0 – LoRes (Enhanced ULA) mode, 128w x 96h pixels, 256 colours total, 1 colour per pixel
  • Layer 1,1 – Standard Res (Enhanced ULA) mode, 256w x 192h pixels, 256 colours total, 32 x 24 cells, each capable of displaying 2 colours
  • Layer 1,2 – Timex HiRes (Enhanced ULA) mode, 512w x 192h pixels, 256 colours total, only 2 colours on screen
  • Layer 1,3 – Timex HiColour (Enhanced ULA) mode, 256w x 192h pixels, 256 colours total, 32 x 192 cells, each capable of displaying 2 colours

Layer 2

  • Layer 2 – 256w x 192h pixels, 256 colours total, one colour per pixel

Layer 3

  • Layer 3,0 – Text mode, 320w x 256h pixels, 256 colours total, 40 x 32 cells each capable of displaying 2 colours
  • Layer 3,1 – Text mode, 640w x 256h pixels, 256 colours total, 80 x 32 cells, each capable of displaying 2 colours
  • Layer 3,2 – Graphics mode, 320w x 256h pixels, 256 colours total, 40 x 32 cells each capable of displaying 16 colours
  • Layer 3,3 – Graphics mode, 640w x 256h pixels, 256 colours total, 80 x 32 cells, each capable of displaying 16 colours

This is all on page 116 of the first edition manual.