The table at the top of Page 16 of the rules gives sizes of units.
In a Roman army I presume that I would take the size of a unit of skirmishers to be the same as that of 2 deep infantry, ie a small unit of archer skirmishers would be 8 - 12 figures.
However what do I do for Ancient British slinger skirmishers? Do I take the 4 deep infantry size (because that is what their warbands are in) which would make a British small unit of skirmishers 20 - 24 figures, or do I take the same 2 deep infantry size as the Romans (8 - 12 figures), on the grounds that skirmishers are likely to be spread out in a relatively thin formation, regardless of what how their close order troops are formed, otherwise they cannot use their slings, bows etc?
I have used 8s based individually. That way they can be deployed in a flexible way. Usually they will just be strung out over the same frontage as a Standard size unit. Generally that is how it works out - a small skirmisher unit or a small light unit in open order covers roughly the same frontage as a Standard close order unit.
You could base your skirmishers and light troops on fixed size multiple bases but it limits your flexibility. Thats why i went for the individual approach.
Yes, I was also planning to base my skirmishers individually and space them out (like the picture of slingers on page 20). It was just the question of how many I would assume make a small unit and whether this would be different for Romans and Ancient Britons. If it is based on frontage then it is clearly the same (8-10 figures) for both, not 20-24 for Ancient British.
The frontages for formed Standard units and Small units in open order should all be similar, for the reasons discussed earlier. Generally a Small unit will have around half as many figures as a Standard unit in two ranks, but that is entirely up to you.