The largest that will fit. That sounds like a smart-ass answer, but it's not meant to be. I'd figure 3/4- or 5/8-inch. It seems expensive when you shell out that money and it all arrives at your door in a box smaller than you brought your last pair of shoes home in, but in the grand scheme of things, it's a drop in the bucket. -Brad
Not all shperical rod ends are made equal. Look for quality manufacturers like FK and Aurora, and make sure you're getting their better lines (they make cheap ones too) I don't like greaseable ones, I'm lazy, and I won't use it (that's my opinion, take it as you will). I like the Teflon coated ends with a hardened ball (often CrMo, or some other zooty alloy) Cheap joints will get sloppy in a very short time, and your performance and handling will get scary to say the least. All that being said, how about some pics of your custom links? Let's see where your hard work is going.
FK is what I run with high misalignment on my buggy. Great joints and sweet pricing too...granted I am running both 1" and 3/4x3/4.