welcome to GoY,
no not at all. as long as there are 2 or more to allow cross pollination the size doesn't matter. They don't have to be planted next to each other either but if too far apart the bees etc may not 'find' them.

I only have one plant, and it does produce berries so maybe having two is not essential (or maybe a neighbour has one?).

Judging by the bees in my area, the plants could be anywhere within 30 meters of each other

I had a couple of hollies separated by about 18 ft. one always berried really well and the other hardly any. the male plant was pretty much in the middle. I can only assume the bees followed a specific flight path and ignored the other one quite a bit.
the neighbour's holly benefited from my male one.

Thanks so much for all the comments,fingers crossed for berries this autumn!