Frame Size and Function
There are several parameters of such a communication protocol that could be determined arbitrarily by the designers, however, these parameters do have some interdependence. Therefore what they do is they write the equations between the parameters, and see what combination results in all parameters being within an acceptable range.

There are basically two main reasons to limit the maximum packet length:


so that a reasonable amount of memory would be sufficient to implement the HW (these days 100KB is nothing but 30 years ago putting 100KB of RAM into a single Ethernet card would make it pretty expensive, and DMA-capable HW was also rare and expensive at that time),
to give other stations a chance to talk (the longer a single station can hold the communicational channel and the more stations you have, the longer it could take for a station to get any message to the channel).