[Plugin: Really Simple CAPTCHA] Change width of CF7's Captcha input field?

Yes.

Example
[captchar captcha-778 4/4]

See https://contactform7.com/2009/11/25/captcha/

That was the first thing I tried but field is still wide in all browsers.

I have:

[captchac captcha-87]
[captchar captcha-87 4/4]

Here is the page:

https://med-list.info/research/