The Sims 2 Ultimate Collection came with a fixed 800x600 screen resolution that could not be changed, at least not if you have one of the latest graphics cards - that's because the game does not recognize these.
I myself was one of the unlucky ones. Googling to see if this can be fixed in any way I run to a relevant article posted at Sims VIP by Wee Albet.
Article includes step by step instructions on how to increase the screen resolution. As the procedure is based on modifying files, it is highly recommended to back up the involved files before any modification.
Read how you can enjoy a better screen resolution at Sims VIP site.