my first big hit game was a starship flight simulator.
you've taken the first step.
you've defined the scale of the simulation. 1 d3d unit = 1 meter.
the next step is to define the maximum near plane distance you can get away with when drawing up close, perhaps something like 10 meters.
then you have to determine the size of the biggest nearby thing you'll draw. i believe a red giant star will be the biggest.
then you have to determine how small it will be before you don't draw it, say 10 pixels wide. smaller than that, its too far, and you cull.
the size of the biggest object, combined with how small you draw it, will determine max visual range. given a 45 degree vertical FOV, and a 16:9 aspect ratio, the apparent size of an object will be approximately width / distance. IE a 500 meter long starship at a range of 500 meters will have an approximate apparent size of 1 meter across (ie about twice as wide as the monitor, say 4000 pixels). its not until you increase the distance to the point that that 1 meter (~4000 pixels wide) shrinks down to 10 pixels or less that you can think about not having to draw the object because its too small. the distance that your biggest object goes below 10 pixels or whatever size you decide on will be your max visual range.
so now you know your near plane (10 meters) and your max visual range, your far plane (a sphere with a diameter of a red giant, drawn far enough from the camera so its about 10 pixels wide - however far that is).
if those 2 numbers fit in a zbuf, you're done, if not, you draw in passes, far to near, setting the clip planes and clearing the zbuf before each pass.
to calculate the max visual range:
lets say you want a red star to be 10 pixels wide at max visual range. 10 pixels wide is about 3 millimeters wide on the screen, or 0.03 meters.
so red giant width / max visual rage = 0.03 meters
max visual range (your far clip plane) = red giant width (in meters) / 0.03
as you can see, this will be a very large number. about 33 1/3 times the width of a red giant (in meters).
needless to say, that's too big for a regular float based d3d coordinate system.
so you'll need a world coordinate system to model the game with, and then you'll need to convert to camera relative coordinates just before frustum cull and draw.
a float gives you about 8 decimal digits and one decimal point to work with. so you can represent numbers up to say 9,999,999.9 or so, with one decimal place (10 cm) accuracy. or you could go up to perhaps 10 million meters with an accuracy of 1 meter.
i'd probably use an int. that gets you something crazy like -4 to 4 billion or trillion meters, with 1 meter accuracy.
i've played with far clip planes as large as 100,000 or more. i think i tried 1,000,000, but i'm not sure.
you may find that your scale is too small for drawing your most distant objects, without some LOD type strategy. based on what you do, there's a theoretical max size for a float based 3d coordinate system such as the camera uses. the farthest thing you want to draw must fit within that range to avoid some LOD type fix. so if you have to draw things out to 1 billion meters, and d3d's coordinate system can only do stuff out to about 1 million d3d units, you'll need a scale like 1 d3d unit = 1Km, or go with a fixup to draw objects closer and proportionally smaller.