My First Transplant: HELP

This is my Chinese elm. It's been in the ground for 2.5 years and I think it is time to put into a pot. It is 15 inches tall, 13 inches at its widest and the trunk is about 1.5 inches in diameter. What size pot should I get? How do you calculate such things?

I haven't thought much about what I want to look like except that I want it to look natural--just an old, nice, shade tree-- but small. Maybe in a shallow, oval pot.

I assume I should transplant this during winter while is is dormant, correct?

This is my first attempt at bonsai so I appreciate any suggestions, help and advice. Since I have two years invested in this already, I don't want to kill it the first time I do something with it.

Thanks

Regarding "This is my Chinese elm.":
I attached a picture, but I guess you can't see pictures in this forum. For the record, all its leaves are gone and I trimmed away all the small twigs. It looks kind's sad actually....

Transplanting is best done in late winter or early spring. Just before it pops new leafes. In my area we are 2-4 months before that now.

You say your tree looks sad. You should only transplant fully healthy trees. When I dig up trees, I plant them in trainer pots which fits most of the roots, except for those growing downwards which is cut off. I dont see any reason to transplant from ground to final pot, that is better done when nebari and at least 50% of the ramification is done. Proboably a few years from now.

Here is the picture...






Why did you cut off all the leaves and ramification?

I did NOT cut the leaves off. They have fallen off every year by the first 2 weeks of January. In the picture you can still see some on the ground. I DID cut off the twigs attached to the branches however. I was told to do that by my local nursery.

Please tell me what "nebari" and "ramification" mean in this context.

As for "...looking sad." I didn't mean it looks unhealthy. This is southern California and most of our trees don't drop all of their leaves. In the spring and throughout the summer the foliage grows fast and looks healthy, but now it sits in the middle of my front lawn--small, naked and all alone. To me it looks sad. I envision it as a simple but beautiful bonsai, in a bonsai pot, displayed on a place of honer. Then it will look happy to me.

Nebari is the rootbase, the thick roots going in the ground at the base of old trees.
Ramicication: if you look at a tree, you will see the trunc ( in your tree: check), primairy branches ( check), secundairy ( a few) and tertiary branches ( nop, cause you cut them off???????)
If you want to make this a bonsai you will have to grow the secundairy and tertiary branches and get rid of all the ugly straight cuts you made. I also see inverse taper at the end of some branches, that will have to be cut off too.
So, dig up your tree, work on the roots , give it a good prune to adress the things i told you about, and place it on top of a tile ( to create a flat rootbase) back into the ground.
You might want to invest in some of the online courses on this site, they are really helpfull.

I second Lucr. Back in the ground or in a training pot. Depending on how much hurry you have. In ground is better, but beginners like to rush it to pots which will take longer to develop a bonsai. Do not put it in a bonsai pot.

Thank you. You guys are the best!

I will look into training videos. In a few months I will dig it up, put a tile under it and plant it back in the ground. Hopefully training videos will give me ideas on what I want it to look like and where I should cut it back. I'll send more pictures in a year or two.

(Isn't it odd that, the older I get and the less time I have on this earth, the more patient about waiting I have become?)