Tape was/is available on many sizes and types of reels.
2-, 3-, and 4-inch reels for 1/4-inch tape with “cine” 3-spline center spindles was commonly used on inexpensive home recorders. Very finely machined aluminum reels with 0.150 tape were used in the Nagra SN-type “spy” recorder and are quite rare. The consumer tapes recorded on these smaller reels are generally 2-track mono and are typically at 1-7/8 and 3-3/4. These are the most likely candidates to be recorded on “rim drive” (capstanless) recorders. 5-inch cine reels were widely used and came in small and large hub varieties. The large-hub variety was favored as a distribution medium for radio commercials as the large hub would work well on studio tape machines. The small hub reels were used on a variety of portable machines, including the higher-end units from Nagra, Stellavox, Tandberg, and Uher as well as consumer models from Sony and others. These could be mono or stereo full, two, or quarter track. Bruel and Kjaer made an FM instrumentation recorder that used this size reel and recorded four channels simultaneously.
7-inch reels were everywhere. Many of the portable machines listed above could use them with their covers off. This was the largest size reel that many of the consumer tape decks could accomodate. These were available in both large and small hub versions, with the large hub being common for pre-recorded tapes and the small hub common for blank tape so that a half hour at 7.5 in/s could fit on the reel.
10.5 inch reels with NAB center hubs are the most common in radio station and recording use. Radio used many 5- and 7-inch reels as well. Consumer decks sometimes used cine-centered 10.5 inch reels, but the bulk in North America were NAB hubs. In Germany, DIN hub reels were available in approximately this size, I believe.
12- and 12.5 inch reels are less common but were made. Few professional tape machines would accept them. The workhorse recorders such as Ampex 350s, AG440s, Scully 280s, Studer A810s, and A807s would not accept reels this large. Sony APR-5000-series, Otari MTR-12 series, and Studer A80 and A820 in some models would accept this size reel. These became more popular again as 30 in/s came back into vogue as a mastering format in the 1980s and 1990s. In Europe, 30cm reels are common, so some of the Studer models may accept 12-inch or 30cm reels but not 12.5 inch reels.
14-inch reels were the largest reel commonly available. These were widely used for syndicated background music and syndicated radio programming. The 14-inch reel holds twice as much tape as the 10.5 inch reel.
Hubs or pancakes–There are two types of hubs: NAB and DIN. The DIN hub was originally about two inches in diameter and later grew to about three inches in diameter, similar to the NAB hub. Pancakes were usually available in 10.5 inches (NAB), 30cm (DIN), 12.5 inches (NAB) and 14-inches (NAB). The 14-inch pancakes were widely used as source tape for duplicators that would then be wound onto smaller reels or loaded into cartridges or cassettes.