Mike they are 4" sqr and 7"sqr.
I never filled the tank to the top (used to leave it about 6" from top) so never had any problems with water sloshing out.
To answer your question,if you filled it right to the top then yes water would slosh out.
To be honest Mike I just need space in my garage and rather than messing about with ebay I thought i would stick it on here.
At the end of the day mate its a 650 ltr tank that works and as a bonus has an immersion heater fitted.
Its yours for £60 if you want it
.
Thats a massive saving from buying new.
Pete