The Garmin 520 has a stupidly small amount of memory so you can only download fairly small areas. I don’t have a 520 now, but from memory it was sub 50Mb which is REALLY dumb.
Anyway, if you follow the dcrainmaker link above and use that method, you can choose different map segments and it tells you the filesize as you select the tiles.
It’s a bit of faffing around but it is possible to get the maps on the unit. I noticed that sometimes the maps would take a long time to generate but other times it was quicker. I guess it just depends on how busy the server is when you start the process. Maybe try very late at night when there is likely less people trying to build maps??
Also, it’s possible to swap out maps as well, so if you travel to different areas, you might need/want to load a different map set on it. I used to do that quite often.
Thank god I lost my 520 and had to buy a 530 to replace it with much more storage for maps of a much larger area!