Check out this website
http://www.signalbox.org/signals.shtml
You should be able to scale your signal arms so they are about 15 -16 mm wide and therefore be correct size.
I'm not sure exactly which signals you would want for your Thomas layout but a casual glance through should be OK.
If you are having a problem with "scaling" (Remember you should be OO 1:76, not HO 1:87) email me off list and I will do it for you into a word doc or similar!
Regards from Down Under
Trevor
Hi there,
I don't know if you read Model Railway Express Mag (www.mremag.com) but write to Brian Mcdermott as he was using cutouts some time ago... I did try to follow the links but lost it in the passage of time
Email me off list (xdford47@yahoo.com.au) and I'll check my own back emails in case I do have it!
Trevor www.xdford.digitalzones.com FYI
Interiors and people figures make such a difference. Especially the people.