Letter size for quick ideation sketches. A4 if I want them to look nicer (with marker). I guess it depends on my mood. These sizes are easy to scan for presentations.
However, I’ve always felt that A3 allows me to explore more freely with multiple sketches per page rather than feeling stuck/done with an 8.5 x 11 page. You can also sketch bigger and get into more detail. But it is a bit more time consuming to digitize it.