How to configure offline imagery for JOSM?

One technique that I've used before (for mapwarper imagery, which is on a slow server and has a short cache retension policy) is to setup the map on marble, (pre)download the tiles in Marble, and then point JOSM at Marble's cache. This enables pre-downloading an area, and gives you greater control in how long the tiles are kept.

However, you can't setup Bing satellite on Marble, both because of a technical limitation and Bing's TOS restrictions. Try Mapbox imagery instead, which is often just as good or better than Bing, and technically usable in Marble. But you probably have to setup a Mapbox account to get your own access token for use in Marble, as reusing JOSM's access token probably goes against their TOS.

AFAIK you can't just define a polygon and then automatically download the tiles inside with JOSM. I believe you have to set different preferences according to the kind of imagery (tiles or WMS), and you should (not completely sure, but it used to work like this) disable "auto zoom" for WMS.

I do have these preferences values:

cache.geoimage-thumbnails.expire=-1
cache.geoimage-thumbnails.maxsize=120

I also have this one set: imagery.wms.default_autozoom=false but it doesn't seem to work currently (see here)

You should not raise the tile size, but the tile cache size.

One way to use offline tiles is putting them into structured directories on your local disk, following the yourdirectory/{z}/{x}/{y}.png scheme. You can then add this as a layer by pointing to it from the preferences like this: file:///yourdirectory/{z}/{x}/{y}.png on unix-like systems, or similarly on Windows: file:///c:\yourdirectory\{z}\{x}\{y}.png. To get the pictures you can either tile big georeferenced images with gdal or use available tools to download tiles from the web (might not be allowed in many cases).