How can I increase my font sizes without decreasing my resolution? I have a 24" iMac at home which at the max resolution (1920 x 1200) is very comfortable for me to see my desktop and read text.

Now at work I have a 27" iMac with a max resolution of 2560 x 1440 which with my poor eyesight makes text extremely tiny and hard to read.

If I decrease my resolution to 1920 x 1080 it's back to being very easy to read, though it makes text in some instances (most notably in terminal) quite blurry and hard to read.

So, how can I increase font sizes for all applications without decreasing my resolution?
Apple doesn't have an option for you to do that. However, you can try TinkerTool. You can download it at no cost.

Screenshot taken from TinkerTool website.