OK ... Kimbo ... went onto Thai airways site and got the lowdown on luggage allowances. I just didnt want to take my largest one for carry-on and then be told it was TOO BIG! And lucky I didnt either.
"Hand luggage should have a maximum length of 56cm (22") a width of 45cm (18") and depth of 25cm (10") HOWEVER the sum of the 3 dimensions shall not exceed 115cm (45") and the weight must not exceed 7kg."
So ... I then got out the tape measurer and with my largest the 55cm one it ends up being TOO BIG (it totalled 121cm) - lucky!!! My other 2 are just under the 115cm, one is 114cm and other is 112 cm. But ... its confusing because the one that totals 112cm actually has measurements of 56x36x20 - so at a first glance one would assume that it is too big because of the 56cm, but it is smaller in width and depth, so comes in under the 115cm!!??
So ... for any occasional travellers out there like me who dont know all this - I hope this has helped you too. It also says that checked in luggage for economy class is 20kg per person, with no size limits.
Thanx once again to all for your help.

Kitty