What is the Pendrives fat32 boot partition mini size limit?

[Malone]

Hello
Is it possible to make the primary boot partition very very minimal, on the usb-stick? Maybe only some Mb's, with help of GRUB or something else. I assume there is limits that i don't know about.
The purpose is only for booting the stick on many diffrent pc's. With the main distro on a modern file-system partition on the memory stick.

And if it is possible, how should it be done?

I am grateful for tips and good ideas.