I bought a bonsai kit with purple wisteria from Amazon somehow I was able to make them sprout last Friday.
I am following the guidebook but I am still not clear when to repot / transplant them.
Currently there are 4 seedlings in a tiny rectangular plastic pot(about mini ipad size) and each of them is about 1 ~ 2 inch tall.
I googled and found out the seedling should be about 2 inch so is it good time to move on?
And what's the ideal size for this one bonsai tree?
Wisteria is a monster. It is aggressive in growth--in the proper location--outside. You don't give any geographic location information, so it's pretty hard to give you any advice on growing the vine (and it IS a vine, NOT a tree and it behaves as such). Other than grow it outside, everything else depends.
FWIW, Chinese wisteria (wisteria floribunda) bonsai tend to be on the large to enormous sides of size. Chinese wisteria is most likely what you have. They have to be large or enormous to accommodate their huge compound leaves, foot long blossoms and long internode lengths. They can be worked into smaller sizes, but it's a lot of work even for experienced bonsaiists.
I always have a smile for those who actually pay for a wisteria. They grow wild all around me in the woods. The second photo below is one example. Its as thick as my calf. I will probably collect it at some point.
They are an intensively invasive species here in the middle Atlantic states in the U.S. They grow so aggressively, they can gradually knock entire buildings off their foundations if they're planted too close and left alone. As bonsai, they're a lot of work for a small return. They are attractive only for a month or so when they bloom (if and when they bloom--it takes a least 10 years for a wisteria to be mature enough to blossom). The rest of the time, they are an aggressive, grabby vine that swamps the backyard with growth and tries to overshadow everything in it.
Below on the right, are two examples of bonsai wisteria--keep in mind these ARE NOT GROWN FROM SEED. The first photo is of a vine collected and cut back severely by a friend. It had grown in his garden for more than 15 years before he collected it. The other is of the 5 foot tall wisteria bonsai at the National Bonsai and Penjing Museum in Washington D.C. The first photo is an example of what happens when the vines "escape" containers and are left to grow unattended...Wisteria bonsai began their bonsai lives as mature vines dug up from their garden homes, cut back severely (to a stump) and regrown in a pot. Most wisteria bonsai are developed from older stock, as they aren't all that interesting when young. They get craggy and aged in the garden. In a pot from a seed they're mostly a bother.
