Depending on the type you want to build Bud google WW II tanks on bridges to get an idea of scale. It really depends on whether it was a rail bridge which was favoured by tanks to support thier weight or a road bridge. And since you will want it to accommodate a "modern" tank too, (nudge, nudge, wink, wink) which ar 10 to 12" wide, I would make it at least 18 to 20" wide and 24" wouldn't hurt...
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"There are things in Russia which are not as they seem..."
Georgy Konstantinovich Zhukov
Too many tanks, not enough time....