OAM consists of 544 bytes, organized into a low table of 512 bytes and a high
table of 32 bytes. Both tables are made up of 128 records. [...]

The record format for the low table is 4 bytes:
  byte OBJ*4+0: xxxxxxxx
  byte OBJ*4+1: yyyyyyyy
  byte OBJ*4+2: cccccccc
  byte OBJ*4+3: vhoopppN

The record format for the high table is 2 bits:
  bit 0/2/4/6 of byte OBJ/4: X
  bit 1/3/5/7 of byte OBJ/4: s

The values are:
  Xxxxxxxxx = X position of the sprite. Basically, consider this signed but see
      below.
  yyyyyyyy  = Y position of the sprite. Values 0-239 are on-screen. -63 through
      -1 are "off the top", so the bottom part of the sprite comes in at the
      top of the screen. Note that this implies a really big sprite can go off
      the bottom and come back in the top.
  cccccccc  = First tile of the sprite. See below for the calculation of the
      VRAM address. Note that this could also be considered as 'rrrrcccc'
      specifying the row and column of the tile in the 16x16 character table.
  N         = Name table of the sprite. See below for the calculation of the
      VRAM address.
  ppp       = Palette of the sprite. The first palette index is 128+ppp*16.
  oo        = Sprite priority. See below for details.
  h/v       = Horizontal/Vertical flip flags. Note this flips the whole sprite,
      not just the individual tiles. However, the rectangular sprites are
      flipped vertically as if they were two square sprites (i.e. rows
      "01234567" flip to "32107654", not "76543210").
  s         = Sprite size flag. See below for details.

The sprite size is controlled by bits 5-7 of $2101, and the Size bit of OAM.
$2101 determines the two possible sizes for all sprites. If the OAM Size flag
is 0, the sprite uses the smaller size, otherwise it uses the larger size.