This is about the largest I have ever owned. 5 1/8" and 54 grams:
https://i.imgur.com/iBOht2k.jpg
I believe I have seen larger, a long time ago, maybe around 6" (?), but that is based on a 'guesstimate' and my recollection.
I currently have 2 young neon jewels (born late January 2017, so currently about 9 months of age) that have grown at a ridiculous rate and I believe are now over 5" (but I think would be under 40 grams ). I'll know next month their sizes exactly. If they continue growing like this, they will definitely end up being very large!