It's a small chip connected to the motherboard via either the LPC bus or the SPI bus. There is no constraint on size except the bus dimension and common sense.

If you want to draw a PCB, you can! Just like you can make your own USB-to-SATA board with an ASM1153 though, all you do is draw a board to connect a chip to the outside and give it some supporting crystals. An example TPM chip is SLB 9665, and you can get its datasheet here: https://www.infineon.com/dgdl/Infineon-data-sheet-SLB9665_2.... (if link goes stale, use Google)

As for source code: hardware being closed off is not exactly news (otherwise people would be a lot less angry at firmware issues). There are open-source TPM emulators in use by qemu, so you probably could convince a Raspberry Pi Zero or whatever to pretend to be one by using an emulator and speaking over SPI. There's also a report of TPM on a ARM+FPGA board (https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=...) if that's what you want.