If you thin your Longan panicles when the fruit are about pea sized, you can significantly increase the size of the fruit. Thin them by removing half the fruit in each panicle.
With proper fertilization, you can increase the flavor/sweetness of your fruit. For potted plants, I’ve found out that frequent diluted feedings significantly helps with growth and flavor of the fruit although I have not fruited Longan in a pot before. I have noticed that that giving my Longan plenty of water while holding fruit will give me larger fruit but the flavor is not as sweet nor concentrated. What I do is water a lot during early to mid Fruit development and then I taper off watering towards the end when the fruit is almost ready to harvest.
The attributes of the Longan( size and taste), are very much influenced by genetics and as you probably are aware, grafted trees are much better in this respect.
If you love Longan, just not the one you’re currently growing, I would suggest you hat rack your Longan and graft it with Biew Kiew and/or Sri Chompoo. If you have low scaffold branches, you can easily graft a few of these branches.
Since you mentioned you grew your Longan from seed, I’ll assume it is tall and Christmas tree shaped. In this case, you may want to cut off everything at knee level(18-22 inches). If you can keep your greenhouse warm enough, you can cut it now but if it gets too cold, you may want to hold off till Spring. After you top your tree, you should see lots of new growth from below your cut.
Select about 3-5 evenly spaced branches and remove everything else. Once the branches are large enough, innarch them if you are not good at grafting or graft the branches if you are good at grafting.
If you are good at grafting, you only need to select about three branches but Longan can be difficult to graft if you are not familiar with grafting.
Simon