I've sprouted kumquat from seed and planted it a small flowerpot. Now I have a stem and two pretty leaves. When do I transfer it to a bigger flowerpot? How big is big enough to have a flourishing tree? Thanks!
If you only have two leaves, those will be the cotyledons, and aren't true leaves. The second pair of leaves will be though, but you need to wait to pot it on until a good root system has been formed. You then move it to a pot that's a bit bigger, then later a bigger one again, till after a few years, it ends up in a 15 gallon pot. While its in the pot it's in now, check the bottom to see if roots are appearing at the drainage holes, or, once its got a few leaves, you can turn it out of its pot to see how its roots are doing.
If you've grown this tree from a seed of fruit you've bought, that's fine, but if you want a good fruit bearing tree, it's better to buy one already growing from a nursery or supplier. Kumquats are grafted onto another rootstock to make them stronger and better fruiting, usually. You may get fruits on your seedling in 7 years or so though.