9 track tapes and block sizes

As others have said, no, "equal size" is not a valid assumption. Even for operating systems that use constant data block sizes, the file labels have a different block size. Consider DEC DOS-11 format, which is used by a number of PDP-11 operating systems. That has a 14 byte file label followed by (usually) 512 byte data records. A very common format is ANSI labeled files. These have several 80 byte file labels, a tape mark, data records that can be any size up to "very large" and may or may not be all the same, then another tape mark and another set of 80 byte labels. Other operating systems may be much stranger. Looking at a CDC 6000 series NOS "deadstart" (system boot") tape, the record sizes of the first 12 records are 486, 930, 936, 825, 945, 960, 930, 951, 141, 486, 1266, 3816. And the largest record size is 3846; the shortest is 6 bytes. Yes, that goes against the "rule" which is really just an IBM convention that blocks smaller than 18 bytes are "noise". CDC also supports "long blocks" in which the I/O for a single block is done in pieces, so blocks can be read or written that are longer than what you would think is the limit from the device limits. I'm not sure how long the longest "long block" is. paul