So I also have this question just like you do and I manage to find the answers, but it was quite a journey.
Firstly, Taco Bearings have aftermarket bearings for ZT knives, but they don't publish the specs, and as of April 2020, this bearings system:
https://tacobearings.com/index.php/product/zt-hinderer/
is still 'under development', and it fits the 0393, 0392, 0609, 0909, 0470, 0606, 0223 and 0308 model, so we can positively assume that all those models have the same bearings size.
Next, I checked Bladeforums and Reddit if anyone knows the size on any of the above. And sure enough, someone on Reddit changed their 0308 bearings
https://www.reddit.com/r/knifeclub/comments/fzjoqi/replacing_the_stock_bearings_on_my_zt_0308_with/
In the comment section, he mentioned that he asked ShepardCC on Instagram the sizes.
In the end, the bearings he replaced the 0308 with is Skiff Made Blad's bearings, 1/4' ID, 0.45' OD, 2 mm ball diameter, but if you want to get bearings from elsewhere, that's the size you're looking for.
