Pinkus,
I like using a 100mm FL lens with my E4 as I get the FOV that I am comfortable with for PCB surveys.
The Ex series have a 45 degree FOV. The Exx series standard 18mm lens has a 25 degree FOV.
Some simple maths suggests that for a close-up lens on the Exx camera to provide the same PCB coverage as the wider FOV E4 setup, It will need an FL of 1.8 times that used on the E4. Or to be more precise it will need a close-up lens of FL 180mm (7.2").
A 7 to 7.5" FL would be close enough really. Some users prefer the 63mm FL lens on the E4. They would want a 100mm (4") lens as tested here. I personally would use such only for investigation of single components. Too small a PCB coverage for my personal needs.
As to diameter. The lens on the Exx is around 25mm diameter. Not all of it is necessarily active however. The FOV is a relatively tight 25 Degrees so it should be possible to get away with a 20 to 25mm close-up lens positioned as close to the cameras objective as possible. My 20mm lens was some distance away and there were definitely losses towards the edges. The main area of interest tends to be central in the image so some vignetting is often permissible. Microscopes produce circular images after all

Cost is the issue with increasing the lens diameter. The ZnSe lenses used for CO2 laser focus are now mass produced, so very inexpensive. Anything else is likely to be from a more specialist supplier for lab use and so more expensive. It comes down to what you need, what you can live with and what you can afford really. I am very lucky to have the large Inframetrics and Agema close-up lenses. They will not be the easiest to mount on an Exx camera though as they are quite heavy. I will likely work up with a ZnSe Close-up lens solution for my Exx camera. A 25mm diameter 7.5" FL lens would be a great start. 35mm diameter would be even better, but that gets expensive.
Hope this helps. Take a look at the two sets of images provided by the Inframetrics and Agema 150mm FL lenses. That is very close to what a 7.5" FL lens would provide. Such a lens us halving the minimum focus distance of the Exx camera so it is basically a X2 magnification factor at minimum focus.
I will post some quick and dirty vignetting pictures I took when using the 20mm diameter lens. That will have to wait until tomorrow though. Time for sleep now

Fraser