There is an ISO standard that defines the basic dimensions of various pen refills. The refill commonly referred to as 'Cross style' is designated as C1 in that standard, with a standard length of 117mm and a diameter of the main body at 3.06mm. The nib area is slightly more narrow - I've seen dimensions, but I can't find them right now.
One thing to think about - there is at least one thread on this board that discusses the situation in which the opening in the nib cone of some slimline kits is too small for 'standard' Cross refills. In that situation, whatever surface treatment you might apply to change the color of the refill could be scraped off. I suspect that the root cause of this problem is poor quality control by the kit manufacturer. An obvious and very simple solution is to ream out the hole using a small drill bit if it is found to be too small.