The 1024x768 resolution has decreased at a quick rate during the past four years and was surpassed by 1336x768 in March 2013
[TPU]. Other larger resolutions are on a slower but steady rise (except my beloved 1440x900 T_T).
According to Sources
[1] and
[2], 800x600 makes up for less than 1% of users as of January 2013. Source
[3] says 1.87%, but that is still very low. The next step up in size, 1024x768, currently makes up for approximately 9% of users. I recommend using 1024x768 if you do not want to go any further.
A 1080p Ren'Py game (e.g.
Rising Angels) will display windowed at 1336x751 on a 1440x900 monitor. At stock settings, I have no problems reading the dialogue text, though it is noticeably smaller. This is comparable to running a 1280x720/800 game windowed at 698x392/436 on a 800x600 monitor (tested with virtual machine). Thankfully, fullscreen black bars were added since Ren'Py 6.11.0.
Note that you should generally increase text size when making larger games and possibly make the textbox less as wide. See attached screenshot. (My font size for Notepad is set to 8.)