Tabletop Simulator comes with its own set of tables for you to use, but they're a bit limiting in terms of space. However, you're able to make your own tables without much effort by using the Gizmo tool. If you don't know how to use the Gizmo tool, we've made a tutorial on that as well.
While this tutorial will use a bit of scripting, no prior scripting experience will be needed to follow along (you're literally copy-pasting one line of code). Before starting to script, I recommend either saving your mod or waiting a minute or so for it to autosave, just in case.
For my example, I'm going to take a boring old chess board and put a black/grey table under it. Make sure you have some kind of table image design all ready to go - what you want the face of the table to look like.
Before starting, click and drag your mouse over every object in your mod, then press L to lock all of them. This prevents your pieces from falling into the void when you remove the table you're currently using.
Then, at the top of your screen, select Objects, then select Tables, then select None. This removes whatever table you were previously using. It also resets your hand zones (but you can move those later).
Next, click on Objects again, then select Components, then select Custom. Click and drag the "Custom Tile" component into your mod. From here, click on the file icon to import your image.
If TTS asks you whether to upload the file to the Steam Cloud or locally, select Cloud. Others won't be able to see the image if you don't.
Make sure Stretch to Aspect Ratio is checked, then hit Import. You now should see a small little tile that will eventually become the table for your mod. Welcome to TTS, folks.
Now, right-click this tile. Select Color Tint. The color you select should be what you want the "side" of your table to look like. Hit Apply.
Next, hover over your tile and press L to lock it. This will prevent the tile from being affected by those pesky laws of gravity.
Select the Move Gizmo, then select your tile. Move the tile so that it's at the top-left corner of where you want your table to be. Rotate the camera frequently to make sure the tile is where you want it to be.
Now select the Volume Scale Gizmo. (Remember, Volume Scale is identical to Scale, except it only scales in one direction at a time!) Use this Gizmo to create your table. If your table isn't exactly where you want it to be, you can select the Move Gizmo again to reposition it. Don't get too extreme with size - TTS tends to freak out when that happens.
You're almost done now! Feel free to unlock everything again (hover over objects and press L). If you accidentally unlock your table, press Ctrl + Z to rewind time and try again.
You may have noticed, however, that you can right-click the table, which can get a little annoying during gameplay. To prevent this, you can invoke the dark powers of scripting.
Right-click your table, select Scripting, then select Scripting Editor. Give the editor a second to load. Your screen should look similar-ish to the example to the right.
Copy the below code and paste it into this editor, then hit Save & Play.
self.interactable = false
If you did everything right, you will now be unable to right-click your table. At this point, you're done! You now have a custom table that you can expand to whatever dimensions your heart desires.
It's possible, however, that something did go wrong. Most likely, TTS decided to reset your table's Z axis scale upon reloading the mod. In this case, you should just be able to select Volume Scale again and rescale the table (our scripting code here doesn't prevent the table from being Gizmoed).
If you have questions, feel free to ask! I can answer them and/or realize I'm very unqualified to be troubleshooting Tabletop Simulator and should probably stop trying.