It's okay - I've found the solution in an old thread, here:
https://www.ecwid.com/forums/showthread.php?t=17299
In case anyone else has the same problem, the first solution (Maya's) worked for me. I'm using the Ecwid Wordpress plugin, so I simply logged into my wordpress dashboard, located the Ecwid "store" page (under "Pages" on the left of the wordpress dashboard/admin area), switched to the "text" view (obviously!) and pasted in Maya's code under the Ecwid widgets.
It's worth mentioning that the first time I tried, it didn't work because I hadn't got the jQuery library installed. I fixed this by following the instructions here:
https://www.ecwid.com/forums/showthread.php?t=11106#5 (Thanks again to Maya - who is extremely good at explaining things simply!)
The only thing I haven't managed to do is figure out how to limit the characters in the "Text Area" boxes - Maya's code only limits characters in the "Text Field" boxes. I expect there's a really simple answer to this... but I'm afraid I don't know it as I'm not great with computers... just learning what I can as I go.
Hope this helps someone like me!