If you're just using loose sugar, try and find some with large crystals.

Growing large single crystals is more art than science.

A third, the largest crystal, fit against the second, a fourth and a fifth.

A mass of crystals larger than my head was visible.

Or it might just be possible that they've never experimented with large crystals.

From a pouch at his side he drew a large green crystal.

He brought a large crystal over to me and held it near my head.

If you want fewer, but larger crystals, then the latter approach is better.

However, it is difficult to grow large crystals of fixed composition.

It was hard to be certain because of the way the controls faced the large crystal suspended on the far wall.