It depends, since it will contain only changed data... so it start small and can grow depending of how much data you change. The default block is 128 sectors or 64KB. Take a look at the following VMware KB article for additional details about snapshots: Understanding virtual machine snapshots in VMware ESXi and ESX (1015180) | VMware KB
The child disk, which is created with a snapshot, is a sparse disk. Sparse disks employ the copy-on-write (COW) mechanism, in which the virtual disk contains no data in places, until copied there by a write. This optimization saves storage space. The grain is the unit of measure in which the sparse disk uses the copy-on-write mechanism. Each grain is a block of sectors containing virtual disk data. The default size is 128 sectors or 64 KB.