Help with Wheels please

The Tortie

I have a 1999 Lunar Ariva GT.

I moved house this year and decided I really needed a motor mover fitted. 

Ordered one to be supplied and fitted by a company. 

The parts duly arrived and the man came. He looked at the van and said he could not fit it because it would be so low it would be torn off at the first speed bump. Apparently the reason is the wrong sized wheels are fitted.

Asked Lunar. They don't have the technical data!

Asked a dealer. Absolute silence. 

Can anybody out there please help? 

What size should they be? 

Thanks