When you know the maximum row length, double the value to calculate the size of your multirow data block definition.
The file system allocates any row that exceeds the current size of a multirow data block to a WAL log data block of its own.
Depending on the configuration of your system, the database builds its data blocks using either native 512 byte sectors or native 4KB sectors. Systems built on 512 byte sectors are called unaligned configurations, while systems built on 4KB sectors are called aligned configurations.