Hello I have a Chinese elm pre bonsai and I’m trying to figure out when is a good time to repot to a bonsai pot?

In the first week of yhe Spring.

It would be okay to pull that jade looking weed any time soon. As for the elm, go ahead, and wait for spring, it just doesn't look like it needs anything done at this time.

What size pot do you think?
And every when after that ?

What about the pruning?

Wait for spring

More details: put it outside, and take appropriate care for your climate zone. That means give it shelter if you get too cold, but not inside.

Sometime it will think spring. This could be April, March, February, whatever. Watch it for new buds wanting to burst out. When you see them, it is time.

I would not plant this tree in a bonsai pot for a few years. There are so much more development to do and that Will be faster in a bigger pot.

Before you decide what to trim, and when to put it in a small pot you should decide what your future image, and especially, size expectation for this tree is. Trees this size can be kept as is in a shohin sized bonsai. But many would advicate to grow it out a little and just use this as a starterplant, to grow out in a larger sized bonsai. Choice is yours. Both are valid approaches.

If you like it the size it has not, and you only want to further the crown of the tree, go ahead and in spring, as you see the buds pushing for the new growing season, plant in a bonsai tray. If however you would prefer to double the size of the trunk for instance, you would be better off getting a larger and shallow pot and plant in that in spring.

In both cases you will need to spread the roots out n a nice evenly radial pattern and cut back any overly thick roots. This will create a nicer nebari, or rootspread, one of the desired features for good bonsai.