The stems have to be long enough to insert into the foam and hold fast - but not so long that they begin to crowd the foam and start banging into each other.
A finger length or a little shorter is usually good. It all depends how big you want your bouquet. The longer the stem - the bigger and more open the bouquet will be.
I cut a sharp diagonal cut so that the stem inserts cleanly into the foam. Be sure the thorns are removed so they don't tear up the foam surface when inserting into the bouquet holder.
Red roses provide such dramatic impact against the beautiful glossy green of the galax and leather leaf fern.
I place the first open rose in the center of the bouquet.