The above comments well discuss the possibilities for most covers of the size #10 and below. That discussion applies to post cards and postal cards as well. The non-mentioned addendum I would add is for mailing tags. They can be fit three to a page thus a 3s, clear or black page works. Some smaller tags and small covers can even fit a four row page.
Now for other size covers and wrappers. Lighthouse and other makers have many size pages. With Lighthouse, the pages fit various album systems. Maxima is their largest can handle 329 x 327 mm (13" x 13" or so) covers.
The larger sized pages are four ring rather than 3. But you can generally put smaller pages into large albums when desired.
One of the larger sizes I like allow for me to slip in an entire Hagner stock page. This allows me to mix different size covers in the 1S (one pocket) pages. While Hagner makes all the usual 1, 2, 3 etc strip pages, it make one other. The is a page for a cover at the bottom, blocks of four in the middle and singles at the top. In short it is a three strip sheet with three different size mounting strip.
Not all covers worth collecting are small. See: http://goscf.com/t/75555&whichpage=2
for a Christmas package (the wrapper is larger than shown in the illustrations). I published an article about the package wrapper. To summarize, the Christmas package was sent parcel post with Special Handling (to speed fourth class transit) from Los Anglese to San Fransisco in September, 1944 constant with the Christmas mailing deadlines. How far west the package was carried from San Francisco is unknown, but from that point it returned east to Washington, DC for sending to the surviving family members of the service man, J.P. Goggins CBM, killed in action. In this case also the sender.
The intended recipient was Chief Boatswain's Mate J.P. Goggins. Boatswain's mate duties are to keep the ship neatly painted, in repair and seaworthy; in short, upright and operational under any conditions
. These are rather continuous but not highly challenging activities normally, except when they aren't. The CBM oversees and directs the ship's BMs. October 25, 1944 was however when the job was most challenging.
J.P. Goggins, CBM was onboard the USS Hoel on the morning of October 25, 1944 when the small group of ships in Taffy 3 (escort carrier task group) stumbled upon the main battle force (Central Force) of the Japanese Imperial Navy with Vice Admiral Takeo Kurita on his flag ship the super battleship Yamato. This, the largest battleship made, had a greater displacement than all of the Taffy 3 ships combined. The Japanese Center Force besides the Yamato, consisted of the battleships Nagato, Kong#333;, and Haruna; heavy cruisers Ch#333;kai, Haguro, Kumano, Suzuya, Chikuma, Tone; light cruisers Yahagi, and Noshiro; and 11 Kager#333;-, Y#363;gumo- and Shimakaze-class destroyers. All were armored to withstand the largest guns of the Taffy 3 ships.
Destroyers Johnson and Hoel with the smaller destroyer escort Samuel B. Roberts, all charged and directly attacked the Japanese fleet. Hoel was the first of the three lost. Hoel both fired at the Yamato causing it to turn and was hit one of the only ship ever struck by shells of the Yamato. Commander Kintberger described the courageous devotion to duty of the men of the Hoel as "Fully cognizant of the inevitable result of engaging such vastly superior forces, these men performed their assigned duties coolly and efficiently until their ship was shot from under them."
Taffy 3 received a Presidential Unit Citation which read, "For extraordinary heroism in action against powerful units of the Japanese Fleet during the Battle off Samar, Philippines, October 25, 1944. ... the gallant ships of the Task Unit waged battle fiercely against the superior speed and fire power of the advancing enemy, ... two of the Unit's valiant destroyers and one destroyer escort charged the battleships point-blank and, expending their last torpedoes in desperate defense of the entire group, went down under the enemy's heavy shells as a climax to two and one half hours of sustained and furious combat. The courageous determination and the superb teamwork of the officers and men who fought the embarked planes and who manned the ships of Task Unit 77.4.3 were instrumental in effecting the retirement of a hostile force threatening our Leyte invasion operations and were in keeping with the highest traditions of the United States Naval Service."
Naval historians considered this battle, the Battle off Samar between Taffy 3 and the Imperial Japanese navy Central Force one if not the most lopsided battles and greatest last stand battle in the history of naval warfare. After that day, the Imperial Navy was never the same.
Taffy 3 attacked so fiercely, that the Japanese though they had stumbled upon a main carrier battle task force.