A. The default cluster size for a FAT partition is as follows:

Partition size Sectors per cluster Cluster size
<32MB 1 512 bytes
<64MB 2 1K
<128MB 4 2K
<255MB 8 4K
<511MB 16 8K
<1023MB 32 16K
<2047MB 64 32K
<4095MB 128 64K

This is why FAT volumes larger than 511MB are not recommended due to the amount of potentially wasted space due to the 16KB and above cluster size.

The default for NTFS is as follows:

Partition size Sectors per cluster Cluster size
<512MB 1 512 bytes (or hardware sector size if greater than 512 bytes)
<1024MB 2 1K
<2048MB 4 2K
<4096MB 8 4K
<8192MB 16 8K
<16384MB 32 16K
<32768MB 64 32K
>32768 MB 128 64K

NTFS better balances the trade off between disk defragmentation due to smaller cluster size and wasted space due to a large cluster size.

When formatting a drive you can change the cluster size using the /a:<size> switch, e.g.

format d: /a:1024 /fs:ntfs