That's a pretty big space you have there.
Like a few have said code 100 is the most often used size, and probably easiest to find used,...particularly as new track has gotten kind of pricey. You might also consider that you want turnouts to match that same size track, and used code 100 is a bit easier to find, and easier on the pocket book. I've found in the past that painting the track (& ties, ....weathering) makes the code 100 seem smaller.
I'm using all code 100 flex track and Peco turnouts on my new layout. But not being a computer track planning wiz, I designed mine with the old drawn pencil. But getting the details to work out correctly in laying track and turnouts ended up much easier and more fun when I started to plan the layout with full size track and switches on brown paper,...loke this
https://modelrailroadforums.com/for...ill-mainline-pick-up.31511/page-2#post-460410
...and this,...
Full Size Paper Templates of Trackplan
those paper templates of the Peco switches were a life saver.
BTW it also helps to have some old junk track ( I term it,...brass, steel, sectional curves of different radi, etc) around to use during track planning. One doesn't mind cutting it up to explore a custom fitting.
Peco turnouts (at least the code 100s) also have the distinction having a curved radius shape to their diverging routes, rather than straight segments at their diverging tracks (not prototypical of American trains and track). But it allows one to get a more compact track plan, and utilize those regular turnouts within curved portions of a plan.
I also HIGHLY recommend the Peco large curved turnouts ( I call them double-curves like the British do). They are rather large radi of 30" & 60" on one single switch,... which at first I thought was going to see very little use,...boy was I surprised at how useful they are.