Try to think modular. You should not make an object which is larger than 1mile x 1 mile.... 5280x5280 units (feet) in ac3d.
You can make one texture repeat over and over in one and same polygon. You just have to make the texture the same way as teh ground textures (= edges have to match teh opposite edges)
There are always 2 ways.. either make a big original object use only a few of those, or make several smaller objects and build tehm as groups in OE. Each case and decision depends on "how good it looks" versus "with how few polygons you can build it". The fewer polygons, the better... but you are teh one to decide a decent appearance.
The custom coast line system I and KAnttori built for teh upcoming 'Karelia' terrain includes 5 sets of differently shaped and textured coastline objects which each can be placed in 4 optional places within the mile square. Their max dimensions span for 1/4 of a mile, so at least 4 of tem are required for each mile sized water square, but they can also overlap in some places and when making a reiver with these objects, even 3 of them may reside in one quadrant. The main thing is that their textures match eachother on the edges. The objects were named according to which edges connected to land: n, e, s, w, ne, nw, se, sw and xx, which connected to all 4 sides. The xx object was needed to fill the empty quadrant when the actual coast line goes in the other 3 quadrants.
512x512 texture is adequate for covering 1 square mile of ground, even though the normal terrain tiles use the same resolution for just one quadrant.
I used 1024x1024 textures for a custom city shoreline. There I had 4 simple square polygons (size 2640x2640 units) and placed them in each quadrant of the mile.
The more complex models, like planes, vehicles and ships are not available in the OE. That is quite reasonalbel, since they have required lost of work, are made by HTC and could be easily stolen with teh easy ac export and to dxf from that.
I would also recommend putting the functional objects like tower, officer clubs, acks, shore batteries, etc aside for now. The export function in OE does not seem to export all the necessary funtional tags to ac files. When you convert tehm back to OE, they are visible, dont work anymore. I have tried this with a vehicle base tower object.