Tim: One possible layout for an equestrian arena is the 6-frequency icosa geodesic dome with seven rows of triangles, counting from apex to ground (sometimes called a 7/18ths truncation). For the sake of argument, let''s say the maximum length of steel-tubing strut you can use is 12 ft. In that case, the spherical diameter of the particular dome design I have in mind could be as large as 94.853 ft.. The floor or footprint diameter would then be 89.708 ft. and the dome''s overall height would be 32.019 ft.
But 12-ft. struts are probably too long for this kind of dome (with soft covering). And you also mentioned your target diameter is 100 ft. In that case, you might have to go to a higher frequency geometry, possibly 7 or 8.
Whatever you decide on, it seems this is a very large project, both in terms of dome size and costs, best handled by a professional dome-building company.
I suggest you contact Blair Wolrfram at Dome Incorporated in Minneapolis, Minnesota, USA. He has built hundreds of large pipe domes and hub-&-strut dome over the years, including two domes for housing climate researchers on Northern Greenland''s glacier (an international project run by the Univ. of Copenhagen, if I''m not mistaken). Blair also owns horses.
Gerry in Canada