i don't think it will change much, because capsule size 1 will still contain approximately half a gram of powder (it also depends on mesh and density), so you will still need to dilute the active ingredient 5/6 times, between diluting it 5/6 times and diluting it 6/7 times I doubt it will change much.
if u had access to a pharmaceutical mixer then the size won't matter, it will mostly be a matter about time.
I would probably look more into making a solution or even a suspension rather than try to dose correctly the capsules.
If u really want to nitpick, if u buy a scale with a 1mg resolution (which should be enough, there are scales up to the tenth of a mg, but they cost hundreds of dollars/euros) u can measure each dosage and put each weighed dosage in each capsule, for that purpose bigger capsule will be better, but it will take a lot of time.