You are able to find unis with bigger wheels than 36, and rims for penny farthings in different sizes can be found on unicycle.com etc. These might be designed for solid rubber tires, however.
Since you are an unafraid inventor, I would suggest checking out this thread where @Ulkicycling reshaped and spliced two smaller rims into a bigger one:
Well, some issues with the actual number of spoke holes might arise, when I think about it. I am unsure if that was addressed in the post.
Oh, and people have been splicing tires as well to custom sizes. You can also cut down and rethread longer spokes, getting basically any wheel size you like from 54" and below with easily available parts.
There exists custom complexe unicycles like the Red Menace (search the forum) with a full range of gears. Also, there might be rumors about compact Planetary Schlumpf like hubs with 3 gears.
On a side note: You can simply highlight text from an earlier post, and you will get the option to quote it like I did in this post. That will also notify the one you are quoting, and link back to the quoted post.