How do I recover a beetle where I loaded a program that was too large and overwrote the bootloader?
I don't know the Sloeber toolset, but it seems very unlikely, that it really has overwritten the bootloader. When programming via USB, it is actually the bootloader, which writes the program into the flash memory. So that would mean, that the bootloader had overwritten itself, only because a too large program. Not sure, if this is even possible.
Anyway, you can utilize an ICSP/ISP programmer (or another Arduino, if you don't have one) to burn the bootloader onto the chip again. Google for Arduino ISP to get instructions for it, there are plenty of tutorials.
You need to connect the correct pins on the Beetle with the ISP programmer. According to this site, you should find the ICSP pads at the back of the board (the group of 6 pins in 2 rows). Through these pins you can burn the bootloader.
If that doesn't work either, the problem lies somewhere else. As I already stated, overwriting the bootloader because a program is too big for the MCU should not happen and it is highly unlikely, that this is really your problem. You might have damaged something on your board, though we cannot tell that from the description. So you could just try to burn the bootloader and see what happens.