I use anywhere from a 2.5mm to a 5mm. Mostly depending on time of season, day, mood of the fish, and how eager I am to tie on another size. Also depends on what rod I’m using. For my TUCR Precision Noodle’s I use 2.5mm-4mm and on my TUCR Bullwhip I don’t use any tungsten jig under 5mm.
I’d start by getting a variety of 3mm-5mm jigs (maybe 10 or so) with most of them being 4mm. I’m a big fan of Fiskas jigs and prefer to order from them direct but there are a handful of quality jigs out there. Quality tungsten jigs will be relatively expensive though. If someone wants more than $3 for a 5mm jig, look elsewhere.
Also, to really utilize tungsten jigs to their fullest potential, you must remember that since they are heavier than lead jigs of a similar diameter that means you are able to fish a smaller tungsten jig with the same reactionary cadence as a larger lead jig. What tungsten jigs really do for us is they allow the angler to offer a smaller profile while keeping the weight, action, and response of a larger “lighter” profile.
When comparing tungsten to lead jigs, don’t think of tungsten jigs as “twice the weight at the same size” as lead jigs. Think of them as “half the size with the same weight” as lead jigs. That’s where using them for finicky fish can really pay off because you’re thinking about it differently. It’s all about jig control at minuscule sizes. When we are trying to imitate a bug that is virtually weightless, how do we even begin to do that successfully with any jig?! Well, tungsten really helps with that dilemma and if you think about tungsten jigs like I said above, your brain will have an easier time discerning the proper applications for those tungsten jigs when transitioning from traditional lead jigs. Believe it or not, but there IS a learning curve. It’s slight and often overlooked but it’s an important one if your goal is become a better angler by understanding what your gear is actually doing for you.