https://www.gpd-minipc.com/ has all kinds of models for the "tiny laptop" form factor as long as 7 inches is tiny enough for you. Their recent products seem to be pivoting more towards the "handheld gaming computer" space, but their Pocket 3 and MicroPC 2 seem to be pretty close to "pocket computers". As far as I know, these are plain old amd64 platforms that will run Linux as well as any other Chinese motherboard.

The small+portable nature of these phones make them unsuitable for amd64 chips (so far) so everyone is using ARM chips, which means dealing with weird and quirky bootloaders or hard-coded OS keys. Qualcomm is putting effort into getting some iterations of their hardware into a well-supported state, so hopefully we may see better mainline Linux support on their chips soon. However, you're not going to get your hands on Qualcomm chips if you don't beat their (high) minimum order quantities and these tiny keyboard phones are hardly mainstream devices, so they often end up with MediaTek chips which have absolutely terrible mainline Linux support (and even worse bootloader quirks).