Hi, I still remember that some entries of the previous BASIC jams are windowed mode games(4:3).

For windowed mode game: a game area with the 16:9 ratio may be better because nowadays over 60% of desktop computers is with 16:9 resolution.
61.14% = (23.63+16.92+11.01+6.12+3.46)%
( https://gs.statcounter.com/screen-resolution-stats/desktop/worldwide )

Furthermore, a bigger game area may be more comfortable for players. Therefore, if your entry is a windowed mode game, a game area in 1280x720 may be quite ideal. (I said this because the game areas of some entries of the previous jams are with small game areas(640x480, 800x600, etc.))