Many thanks for your quick response. Although your answer did not help directly, it put me onto the solution. Whilst looking for the option to 'select mm' (not available) I came across the option to 'Combine objects'.(please see attached) Happily this solved the problem immediately and Cura accepted my model.
I have constructed a model in 123 D design and obtain the message above (from CURA) when attempting to load into CURA I am using Ultimaker 2+ The model can be contained in a block about 20mm x 40mm x 30mm (x,y,z) and has a minimum wall thickness of 1mm although generally 2mm. I have been unable to find any references to minimum size from on-line manuals etc. so am now trying here for help. Any advice much appreciated.