We've now bought 4 posters. Originally Mrs. P bought a 32x48 international and I got a 32x48 domestic. Then I got a 20x30 international, and most recently an updated 32x48 domestic.

Based on your map, my advice is to get a 32x48 international. Mrs P. has 700+ flights, and I just looked at her poster, and you can easily make out the domestic flights in addition to the international. (On my 20x30 international, the US is almost solid yellow, but I can see the international routes. Note that on the 20x30, the airport codes do not show up.)