What kind of pan for 15 inch beef roast?

I have a very large Chantal covered roasting pan I use for such items. Not sure if it is still available from the company. You could try googling it. Or you could use a large rectangular low roasting pan (stainless steel, likely) and use heavy duty aluminum foil (shiny side in) for the cover.

Granite Ware makes a 19" oval roaster that might serve your needs. Very reasonably priced and will give you years of service:
http://www.bedbathandbeyond.com/store/product/granite-ware-19-inch-oval-covered-roaster/1016902640?skuId=16902640&mcid=PS_googlepla_nonbrand_cookware_&adpos=1o3&creative=43742634349&device=c&matchtype=&network=g&gclid=CK-7nJn07ckCFYcBaQodm2IELw

I would consult the Flintstones. But if cell reception is bad, maybe we could work it out ourselves.
But seriously, Le Creuset or similar quality pans are an investment. Do you expect to be making this cut of meat often enough and long enough to make the cost, space in your cupboards, etc worthwhile?
If yes, a quick search at Amazon shows roasting pans (steel, granite ware, etc) that size and larger, some with racks, some with lids.
If this is a one-time occasion or you're not sure, consider roasting it in one of those disposable foil pans available at grocery or hardware stores, with the pan always resting on a sheet pan (for safety in moving the hot dish).
Either the store bought or the disposable pans can be fitted with an aluminium foil tent if you want to conserve liquids, control roasting time, prevent the top from drying out.
As for the searing the meat, do you have a large griddle that could help?
Last, is this from a butcher and could you/do you want to have the long roasts cut in half?
Hope this all helps.