It is of course obvious that the size of a bead's hole determines what projects it is suitable for. Pearls with tiny holes are suited to being strung on silken thread, and seed beads too can be threaded on whisper-thin nylon. Lampwork beads are available in a variety of hole sizes from around 1mm right up to 5mm and beyond - so what is it that determines the size of the hole?
Lampwork glass beads are wound on mandrels, which are lengths of stainless steel rod. In fact it is the mandrel which makes the hole in the bead - rather than making a ball of glass and then drilling a hole in it, the process of lampwork glass bead making starts with the hole, as the glass is wound around the thing-which-when-removed-leaves-the-hole-behind! Yes, there are perhaps less clunky ways of wording it, but that's how it looks to me.
When using heat and gravity - and indeed as I do, only heat and gravity - to shape a glass bead in the flame, the proportion of glass to mandrel is extremely important. The bigger the mandrel in relation to the amount of glass wound, the narrower the bead will be, unless you wind a much wider footprint of glass in the first instance (which is of course an option). This is why the glass charms which make up some of the range offered by the widespread commercial bracelet systems are very narrow - they're what I'd describe as 'tall and thin'.
I usually stick to two sizes of mandrel here in the Let Fire Inspire workshop - 1.6mm for my smaller beads, and 2.4mm for my larger beads, and to teach with. When teaching my courses, I find the 2.4mm mandrels make for a more straightforward experience in the flame for beginners because they can be grasped firmly and can take a lot of heat, so are more forgiving than their narrower relations.
Most of my beads are made on 1.6mm mandrels quite simply because I prefer the shape of the end product. With a narrow (1.6mm) mandrel the relationship of glass to mandrel is more generous, and the beads naturally end up broader and less straight-sided than their 2.4mm-mandrel-wound counterparts.
Here's what I mean, using the very simplest beads as an example. These are small, plain 'spacer' beads.
The vast majority of my beads are made on 1.6mm mandrels, because:
I prefer the shape of the beads.
The canvas for any surface decoration is a broader one.
In my experience designs are less likely to distort during the process of shaping in the flame. Narrow beads have tighter curves, and some surface decorative designs can't hold up to the physics of the flow of molten glass over these steeper arcs.
In my wireworking projects I much prefer working with smaller-holed beads, both aesthetically and in practical terms of 'anchoring' a bead on wire using tiny saucer beads or rondelles.
I appreciate that there is an important consideration to bear in mind in terms of hole size, in that having smaller holes restricts the choice of stringing material to something that will definitely fit! A bead made on a 1.6mm mandrel cannot, for instance, be strung on 2mm leather cord.
Bearing in mind the shape of the bead you're after, taking into account what the finished product will be, beadhole size is simply a matter of personal preference.