Sambucus does tend to be 'leggy', so I would be inclined to reduce the stems by one third (down to just above a leaf axle) and cut out any crossing branches or ones that spread out from the bottom as sometimes young plants tend to do. This treatment will help to make the shrub more bushey.

it would depend on how high you want them to reach. I have a group of 8 Sambucus nigra 'thundercloud' which i don't really want to get any taller than 6 foot. As a result, i cut them back each year to about 12" high. You can be as brutal as you like with Elders

Remember if you cut all the stems right back you won't have any flowers though.

Another option is to remove half the stems at ground level, and reduce the remainder bya third or half, all done in winter when the plant is dormant. You can also cut it to the ground, but as Steragram says,you will lose all the flowers next year if you do.

Loads of flowers on mine each year and they're cut back to max 12". Flowers smell slightly of lemon too.

Hi and welcome HCtoon I have one in front garden and its stunning and never prune hard back only give a little tidy trim as dont want it getting too tall.

Most of all enjoy the beauty.