iMac 24 inch for App Development

Is a 24 inch display frustratingly small (as compared to a 27 inch) or is it acceptable?

Smaller and noticeable, sure. Frustratingly small, I would not say so.

One can develop on 15" MacBook, even not comfortable). That means that 24" is quite acceptable.

However, as a personal experience:

• when working on MacBook, I attach a second screen (27")
• I had recently to change my iMac 27" which had become obsolete. And no way to get a new iMac 27".
• I did not want to go back from 27" to 24", so I got a Mac mini with 27" display.

I would probably have made it with 24", eventually adding a second screen ; but I preferred the other option.

You should compare the price of iMac 24", iMac 24" + 2nd screen and MacMini + keyboard/mouse + 27" Screen and decide depending on your budget. My advice: whatever you choose, get at least 16 GB memory and 1 TB storage (better 2 TB).

Hope that helps.