I am really starting to notice that my testicles are smaller in size which is great by me. At times they can really really ache an feel kinda like static electricity if that makes any sense. Did anyone have an experience like this and does it go away?

 Yes I have had both. Shrinkage in my case is not the best news as i find it harder to tuck.
Pain does subside over time though.

Can't say I've noticed anything pain wise but after two years there has been a fair bit of shrinkage.

Mine had periodic episodes of dull pain, combined with the "electric" feeling you describe. I think is a reaction to the HRT causing them to "shut down". For me, it started a few months into HRT, and stopped a few months later. I rarely experience anything now except if one of them gets semi-twisted due to physical activity while tucked, and even then it's not really pain, more like "oh, something happened down there".