The reels that I store are 10.5 inch with boxes, both 1/4" and 1/2" tapes. They are stored vertically. Since I have a large number of them (approaching 1200 reels at last count), they take up a lot of space. However, since they are smaller than LP's, they can fit into bookcases, where LP's cannot fit (either in depth or height). I have been displacing books and records from various cases both in my music room and in adjacent rooms as my collection has grown. Most of the tapes are in a multipiece sectional bookcase which wraps around a corner - very nice looking rosewood veneer - which has glass doors, sort of like a cabinet. (I don't use my Rackit racks for the tapes - since those racks use two crosspieces to support the records and the tape boxes can slip between the two pieces of wood.)
I don't have any 30ips tapes, but the ones I have borrowed from a friend who was an Ampex engineer, are in the same size reels - mostly 1/2" tape, so they are the same size as my other 15ips 2 track tapes. They are usually several reels, since 30ips 2 track tapes typically last less than 16 minutes if they are on standard 1.5mil thick tape.
I do keep my small collection of 7.5ips 2 track tapes in a separate shelf of a bookcase. My 500 or so 7.5ips 4 track stereo tapes are in another room, crammed into bookcases, not well organized, except that the labels can be easily read. I don't play them much.
Larry