What is the package size?

I read about unmanaged code in C # and came across the following line: -

In a C # program, the CLR finds the offset using field tokens.
C field names are directly compiled into offsets. For access speed, each field is placed with an offset that is a multiple of the size of the field.
However, this factor is limited by the maximum number of bytes x, where x is the "packet size".

My question is what is the package size?