Describe your suggestion

More useful errors when the build command fails instead of "unable to do it", for example, saying if there is not enough height available, or a list of missing blocks for the schematic

And a simple form of size preview, possibly in the form of a 2D bounding box to show the footprint of the schematic

Settings

An X and Z bounding box (Y wouldn't be needed if my height error suggestion was implemented) that can be enabled/disabled via a "buildfootprint" command (better setting name pending)

Context

To assist in placement of schematics using the build command

Agree on that first point.

However imo it would be enough to have the "size preview" in chat like the starting position.
You will have to do a little math for correct positioning anyway. And this way it wont easily break rendering...

Also I'd add a 3rd request: Building block list.
This could be integrated to show which blocks and how many are missing when its "unable to do it" aswell.

Maybe looking something like this:

i just realised that this is pretty much the whole point of schematica and it's integration...
but i'll leave this open since then it just makes baritone even more useful standalone

Also it should not only use blocks in hotbar also use things in my Inventory.

you can always enable "# allowInventory"

Edited/updated my original suggestion(s) to be more reasonable