A 4 X 8 sheet is a very minimum for HO. It permits
a 22" radius but barely. Simply adding a foot or two
at each end would improve the situation.
The reason radius is important, you cannot run the large
6 wheel truck locos and many steamers on curves with
a smaller radius. The figure 8 in your posted layout
would be nearly impossible and could accomodate only
small 4 wheel locos due to the radically small radius.
Since you are going in with DCC, I would also agree that
you should consider a single track main. You know you can
run two trains on the same track at the same time with
train A going clockwise and train B going counter
clockwise. This does require 2 or more passing sidings
so one train can park as the other passes. This makes for
more interesting operating. Another reason for going
with single track main is again the curve radius problem.
The inner of a 2 track main would have much sharper
radius and again limit the type of locos that could run on it.
Now you could add to your continuous running interest
by creating a figure eight by using 4 turnouts
to connect the crossing with your end curves. This,
however would create 2 reverse loops that would
require 2 reverse loop controllers.
If you haven't already done so, by all means check the
various layouts in our layout design forum. You'll find
two large threads crammed with the layouts of our
members and other layouts that are yet to be built.
You can see certain aspects that you like in one layout, and
other aspects in other layouts. Then see how you can
fit these together to fit your space.