All Ethernet adapters since the first Alto card had self-clocking data recovery [1].
Clock accuracy was never a problem, as long as it was withing the acceptable range required for PLL lock/track loop.
The reason for 1500 MTU is that for packet-based systems, you don't want infinitely large packets. You want small packets. but large enough so that packet overhead is insignificant, which in engineering terms means less than 2%-5% overhead. Thus 1500 max packet size. Everything above that just makes switching and buffering needlessly expensive, SRAM was hella expensive back then. Still is today (in terms of silicon area).
Look at all the memory chips on Xerox Alto's Ethernet board (below) - memory chips were already taking ~50% of the board area!
[1] Schematic of the original Alto Ethernet card clock recovery circuit: https://www.righto.com/2017/11/fixing-ethernet-board-from-vi...
EDIT: Lol! Author has completely replaced erroneous explanation with correct explanation, including link to seminal paper about packet switching. Good.