I took a look at the plan - very nicely drawn, and looks like a great plan for a lot of fun. Unfortunately, it looks like it was designed for N scale, and a lot of features won't scale up very well. Also, it just won't fit in a 12x12 space in HO.
In my analysis I assumed 18 inch minimum radius curves - about the smallest that is practical for an HO 1950s era freight-only layout. If you want to run passenger cars, you'll have to use "shorties" or increase the minimum radius to at least 24 inches (which would be better for appearance all the way around).
In any case, using the bottom wall for calculations, the lower left return loop is 46 inches wide. You need 36 inches for the inner passing track, a 21 inch radius (42 inch diameter) outer passing track, and 2 inches either side for cushion, which gives a total of 46 inches. The doorway should be at least 30 inches wide, then the next loop is 40 inches, the aisle is 30 inches, and the 90 degree turn is 20 inches. This totals up to a minimum of 166 inches, or not quite 14ft. Even if you reduced the aisles to 24 inches - which if you model the OF&S (old, fat, and slow) like I do, you really can't afford to - you can't get down to 12ft.
The plan also has significant access issues when built larger than N scale. Burying the reversing loops at the end of the main in corners is very attractive from a space utilization point of view, but is too wide to reach across in any scale but N. As I said, the bottom reverse loop is 46 inches wide - and that's using 18 inch minimum radius. Normal maximum reach is about 30 inches; can go about 36 inches if the layout is built low. So access hatches are required in the middle of the reverse loops. If you make it a liftout hatch, where do you set the liftout portion when you are using the hatch? If you have a hinged drop-down configuration, you will probably be bumping the scenery with your legs while using the hatch. You also have to plan carefully to make sure your benchwork doesn't interfere with the hatch dropping down. Again, as an OF&S modeler, I don't like access hatches, period. Reality is that reverse/turnback loops in HO almost always require access from at least 2 sides, or access hatches in the middle of the loop.
Given the above, a more practical arrangement for your space might be a once or twice around the walls configuration with perhaps one center peninsula. If you have 2ft wide shelves against each wall, and a 30 inch wide U shaped aisle, the center peninsula could be 3ft wide and protude 5.5 ft from the 2ft shelf. Cheat a couple of inches on the aisle or the shelf width and you could have 40 inches for a turnback curve or reverse loop on the peninsula.
yours in planning