I started work on my Lucid Eye Neanderthals (part painted figures above) today. As you can see they are a bit bigger than the Copplestone cavemen (painted figures above).
The leader, in particular is a massive figure; 33mm foot to eye. Interestingly the two Copplestone figures are also slightly different sizes with the red head a bit larger so that he matches the smaller Lucid Eye figure.
The key thing is that the styles, head sizes and weapon size are a very similar. I would (will) happily mix them together and I am very fussy about non-matching sized figures.