Portrait Android app?

There is no right resolution. If you want your app to run on a variety of devices then you'll need to build it in a way that allows it to adapt to different screen sizes and resolutions. To begin with I'd recommend you to pick the most common aspect ratio for android devices. 16:9 (or in this case 9:16 since you want portrait) comes to mind but don't quote me on it, do some research. After that you should pick a common resolution that respects that aspect ratio (1920*1080 for example) and start designing around it, but always having in mind that it will need to be adapted to other aspect ratios and resolutions later on.

Unity's UI system accounts for this problem and gives you tools to handle it. Anchoring UI elements to the screen's corners and scaling width and height of elements depending on the screen size are some of those. Of course you can always do it yourself by querying the screen dimensions at the start and then setting everything in the right place, it is tedious but it is the only way you can know your app will respond correctly to the different sizes.

Hope this helps!