There's a well-known fictitious law case about a cheque written on a cow.
I think most banks now have contractual conditions allowing them to
dishonour or surcharge for cheques on non-approved forms.

After a mistake in the office earlier (someone printed an A5 letter on
an A0 plotter) I had an idea...

I wonder if you can provide all the details a cheque legally requires,
but printed in such a small font that they need a magnifying glass to
read it - sadly this would affect your ability to sign it unless you
made it roughly normal-size, but with the necessary other text in a tiny
tiny font :-)