The header size is located in the first two bytes of each header.
Example: When you found "GARMIN RGN" before that you see two bytes 7D 00 it means x7D which after conversion to decimal is 125 bytes.
Analog with LBL (xEC-->236 bytes), NET(x64-->100 bytes) and NOD(x3F-->3 bytes)
The only one "odd" is the TRE header that is larger that what you see in the first two bytes, because it could change with copyright info, nevertheless this extra info is not necessary, you could only grab the size that you get from the first two bytes.