Max image width for signature images

[jonimustakallio]

Hello! I am using Roundcube on my hosting provider xetnet.fi and I have noticed since I used MailChimp for Newsletters that the image sizes are not responsive inside Roundcube signature when added in "normal HTML". Is there a recommendation for a maximum of image width that I could use for signatures (as a portrait image of myself) or is there a way to make the image to work as responsive and show on mobile as full width and desktops as max width 640px for example? I guess no custom CSS is not allowed or recommended so any suggestions how I would create it?