I'm looking for portable micro PC (uPC) on a stick (something like Raspberry Pi), but a lot smaller (a USB thumb drive size).Any architecture is great (ARM is fine), as long as you can install Linux on it (so some storage is a must). Support for Wi-Fi is an advantage.Here is which I've found so far: Forget Raspberry PI How About Android on a Stick!, but the article is a bit out-dated.
Portable micro PC computer on USB stick
usb;micro pc
I think the best answer to your question is the Intel Compute Stick.To answer your feature requests:It is small-scale being in principle a HMDI stick (~100x27x13mm)It has a processor that is good enough (Atom Z3735F)It has 8GB (Ubuntu version) or 32GB (Windows version) of memory which is expandable by the MicroSDXC slotIt has 1GB (ubuntu) or 2GB (windows) RAMIt has all the usual Wifi standards (802.11b/g/n + Bluetooth 4.0)If you can wait a few more months / weeks, a new version was announced at IFA 2015.