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I have had a European hornbeam for about a year and have left it to grow. My question is when cutting back (ramification) will the leaves get smaller ?

Yes, for sometime they'll be small, but it will grow big again eventually, this is something people generally do before exhibitions.
Have in mind that the bonsai pot plays a part on that size limitation,
so get smaller leaves is not needed on training trees.

Here's an example on Master Ladoso's Flame Tree...

On his right hand, a finished tree's leaf, on his left hand, a leaf from one his training ones...

is that a Persian silk tree?

No, that's a Flame Tree / Flamboyant...
Delonix Regia.

Thanks for the picture

No the leaves will not be smaller if you just cut back. The leaves will be the same size, ramification is produces by pruning. Allow the growth to extend in spring and prune after the growth has hardened off. The alternative is to do partial defoliation to produce a second flush of growth where there should be more and therefore smaller leaves. However this depends on the age and size of the tree and is not done on young trees that need strong growth

Oh, I thought that by cutting back he was also considering defoliation...
My bad.
But yes, reducing leaf size is the last thing you'll think about on a finished tree.