H.264/AVC/MPEG-4 Part 10 contains a number of new features that allow it to compress video much more efficiently than older standards and to provide more flexibility for application to a wide variety of network environments. In particular, one of the key features is VBSMC.

VBSMC

Variable block-size motion compensation (VBSMC) with block sizes as large as 16×16 and as small as 4×4, enabling precise segmentation of moving regions.
The supported luma prediction block sizes include 16×16, 16×8, 8×16, 8×8, 8×4, 4×8, and 4×4, many of which can be used together in a single macroblock. Chroma prediction block sizes are correspondingly smaller according to the chroma subsampling in use.

luma:亮度. chroma:色度.
可以看出在H.264中宏块最大可以16*16, 最小可以4*4.
其中亮度预测宏块的大小包括: 16×16, 16×8, 8×16, 8×8, 8×4, 4×8, and 4×4.

Perfernences:
https://en.wikipedia.org/wiki/H.264/MPEG-4_AVC