Typically, the link hole and the ball hole fit the same size screw. But if you get m2.5 link ends and the hole is also 2.5mm, you can always drill it out if they're plastic, but I can screw an m3 rod into m2.5 plastic ends. It just takes some force. Just make sure to drill it undersize to give...
If you don't have a set of calipers, they come in very handy in this hobby. If it were me, I'd grab a halfway decent set. Make sure they're metal, and at least IP54 grade. A lot of the cheap Chinese ones in the $30 price range on Amazon should be fine, just look at the reviews. If anyone says...
If you do a Google for "m2.5 link end" you will find what you need. The dedignation refers to the hole size through the ball. You can get them on Amazon or ebay as well. All the major hobby shops should carry them as well. The 3mm hole in the link side will fit a 3mm rod. How are you coming up...