I think you'd need to raise with your support team. There isn't an area you can configure to change the size of the prompts. I vaguely remember some work being done in releases a little time back, so what version are you on?
We are on 7.7.2 and are a SaaS customer so we are pretty up to date.
I suspected this might be the case. Thank you for your advise. I will ask support if it is possible from there end.