Placing a turntable on a Unitrack layout requires space and some precise calculations.
A turntable and associated stalls for a Roundhouse require at least a 4x5' space to effectively model a turntable area. It's best to place the turntable at one end of the layout.
If you use a HCD for bench work (80x36"), you'll have to be satisfied with an area of 3x4'. The radii used at one end of the layout can be a combination of 13 3/4 " and 19" radii. This will give you a radius of about 16 inches in that part of the layout where the turntable is placed.
For turntables, I suggest using the new Walthers 130' turntable and Modern Roundhouse along with Add-On roundhouse stalls. This turntable will allow you to store steam locomotives as large as a Athearn "Big Boy" or "Challenger". Using the 80x36" HCD as bench work you should be able to fit comfortably the Walthers Modern Roundhouse and at least one and possibly two Add-On stalls. This will give you at least six and possibly nine slots for locomotive storage. The turntable is indexed and there is no need for a reverse loop switch as that is included with the turntable.
Walthers had a big sale on this turntable($169) six months ago. That was almost a half price sale. They might reduce the price of the turntable again to those low levels, so keep watching the Walthers ads.
You can use Unitrack sections to place in the Roundhouse and for storage tracks, but It's best to use flex track as the Roundhouse allows for flex track minus the ties to slide into place in each stall. I'm using Peco code 80/55 flex track for this purpose. on the original JJJ&E I used Atlas code 80 track sections and Unitrack for the CMR 135' turntable kit that I built.
Below is a photo of the CMR turntable on the "original" JJJ&E.You can see the Atlas code 80 track sections used as holding tracks around the turntable. These code 80 Atlas track sections were connected to Unitrack connector track sections to complete the length of each holding track. The "original" JJJ&E had two Atlas Roundhouse kits (six stalls) plus an additional nine holding tracks around the turntable. The area use was 40"x60" for the turntable area.
The "new" JJJ&E will have 24 Roundhouse stalls around the Walthers 130' turntable. The radius of the mainline at the turntable end of the new layout is 48 inches, which is more than enough room to accommodate the 24 Roundhouse stalls.
Getting back to the radius you need in the area of the turntable, this radius can also be used on the other end of the layout which will make your layout look more realistic with the wider curves.
If you're not sure how this will work out, lay out the track needed for the curves on the floor. Use 19 inch radiii followed by one 13 3/4" radii and continue this pattern until the curved end of the layout is complete. You'll see that you added the necessary space required for a turntable and Roundhouse at one end of your layout with this arrangement.
Don't limit yourself to a 13 3/4 " radii. You can experiment by using the 13 3/4" radii track section with the larger 19" and/or 28 1/4" radii.
Doing this will help you visualize your needs for a turntable module which will become part of your Unitrack layout. That is the beauty of Unitrack as you can see how track work will look simply by laying out the plan on a floor or on HCD bench work.
In any event,if you're planning to use a HCD as bench work, use the 80x36" size. The additional six inches in width will give you much more flexibility when you plan your Unitrack layout.