I don't know what the Microchip bootloader is compatible with.
But the MPS shows these USB parts with 64K or more.



The chips listed that are not supported in PBP 2.60A, will be added in PBP 2.60B.

Note: They are all 3V parts.

Otherwise, you can make your own FT232 equivalent with any USB PIC chip using CDC.
The 13K50 is probably the smallest @ 20 pins, and is 5V.