Hi,
I do not know how you define "portable", especially when you consider the issue of flexibility (e.g., of either built-in or add-on devices to your main recording device).
I am posing this as an issue worth considering and defining because I have a Zoom
R 16, among many other interfaces, some of them portable, and have used the unit to successfully record piano, which is what the OP wants to do.
I should also say that I recorded piano once, but I did not do a bad job considering my inexperience and the difficulties of recording in a regular apartment - mind you, I got help from people on this forum!
So, here are some questions for you to reflect on (for yourself, but it might help us here to help you a little more):
1. Do you have a maximum size and weight in mind for the recording device?
2. Maximum price?
3. Does using an stand alone / self-contained / all-in-one recording unit (like the Zoom R 16 make more sense than carrying a separate recorder, and perhaps an audio interface?
https://www.boss.info/ca/products/micro_br_br-80/
If it is simply audio you want to capture, and you want something small, you might want to consider the Boss Micro BR BR-80 (8 track recorder), which is about 6" X 3 1/2", and fits into one's shirt pocket. (Can record .WAV or .MP3)
https://www.boss.info/ca/products/micro_br_br-80/
Both the Zoom R 16 and the Boss Micro BR BR 80 can function as stand alone recorders, or as audio interfaces.