I was going to make a poll, but I couldn't really think of a good way to set it up. So, how big of an overworld do you consider too big? This will likely get a lot of different responses from different people with varying preferences, but I'm curious to see what the average ZC player deems as an unnecessarily large OW. This can depend a lot too on what there is to do in said overworld. For example, a standard 8x16 NES map might be too small for 13 dungeons, but that might not be enough to fill a 16x16 OW. It could also depend on what types of areas you have. How you get around, and how interesting/ different areas feel from one another. This will also come down to whether a player prefers OW exploration, or prefers exploring dungeons more.
For me, I don't think any one size is "too big". I think it really depends on what you have going on. If you have some massive map that uses 9 BS maps, but only have 6 dungeons, you better be packing some really interesting stuff in those screens. The inverse is also true. I don't liken it when stuff feels too squished together, or close.