Here's how I understand this... (I'm not an expert in this....)
Higher (smaller) aperature = wider depth of field
Higher aperature = more light needed to take a picture
More light (bigger flash) = slower (longer) flash = more motion in your drops
AND... Distance to target (size of final image)
So you have to trade off Aperature (DOF) vs Light vs Distance to target to come up with a reasonable middle area that gives you acceptable depth of field that's well lit.
If you are photographing simple droplet's like :
Then it's moderately simple.. the DOF is not very wide (depending on the angle you are photographing from) so you can use a moderate aperature. The DOF needed on this picture is probabily no more than 4-6mm.
However if you are trying to take a picture like :
You can see that the required area to be in focus is MUCH wider. If you look in large then you can see that the drops to the side/back are actually out of focus. But as the ones at the front are in focus then it does not look too bad.
How do you do it?
This is a great calculator for figuring out your depth of field using almost any equipment. Put in how far you are from the target, focal length etc and it will tell you how wide your DOF is.. I try to aim for a minimum of 12mm
However with only 1 flash (Vivitar 285) set to minimum power with water I get really dark pictures without dropping down ISO to around 400 and aperature to F13/F14. Milk is a lot easier as it is opaque and reflects the light so even at F16, ISO100 I can get nice brightly lit pictures.
Have a look at the calculator and see what DOF you can get at what distances etc
Take some test shots and see how they come out and work out what you want to sacrifice.
My standard settings (for milk drops)
Canon 100mm Macro :
Distance 34cm (ish) works out to be around 1:3 magnification
F16, ISO 100
55mm M42 lens + 12mm extension tube :
Distance 29cm (ish) works out to be around 1:4.2 magnification
F16, ISO 100
For water drops probabily go to F14 & ISO 200 at the same distances.
You *can* increase the power of the flash you are using.. so you can use higher aperature.. HOWEVER this will mean the flash will be slower and so you risk introducing movement into the final shot.
Try. Experiment, see what works, fiddle around a bit.. and take lots of pics!