Couple things at play here, so I'll try to offer what I think would be the best comprehensive solution.
First, you want a 27-30" panel minimum, as far as size. It will allow for the monitor to be placed at a healthy visual distance without reducing how much area it takes up within his visual field, similar to how a 50" TV looks "smaller" when it's mounted on the wall 10' away from you. You also want that extra screen space to be able to adjust any accessibility software.
Examples of accessibility software for Windows would be the native Narrator software, NonVisual Desktop Access (NVDA) which is free open-source software, or Window-Eyes for MS Office or whatever the equivalent is called now. There are other options out there as well if none of these work for your dad.
Depending on if and how you implement any zoom features to enlarge text, resolution might come into play. If you're going to enlarge text to 150% or larger, I'd recommend a 1440p display over a 1080p. My reasoning here is that as you enlarge text the lines might become blurred or altered due to the ratio between the desired image size and the pixels-per-inch count. In my experience the higher definition displays tend to display enlarged text more clearly.
Finally, I'll link the following thread about programming font options below:
You can also find good lists by searching around the internet if that link dies. I recommend one of these because coders usually prefer fonts that have a high level of distinctiveness and differentiation between characters, which minimizes the chance that you will mistake a 1 for an l for a ] and so on. This is, for obvious reasons, also very helpful to the visually impaired. I believe that there are Chrome and Firefox extensions that will try to apply a given font to websites as well, so those might be worth looking into if your dad browses online a lot.
If you'd like to provide a list of desired specs and a budget, I'd be happy to look around for some models that would be a good fit. Best of luck!