Ah, now I got what you want!
My recipes are:
1. Fit 4 AA where 1 D cell was. They are also a little bit shorter, so you can fit 20 AAs in a 4 Dcell Maglite.
2. You can replace 2 AAs (f.e. Mini Mag or clones) by 3 N cells plus a little spacer. They are also a little bit thinner, so you will perhaps have to wrap plastic film around them (I use 3 N cells in a Mini Mag with 7 LEDs and they give you a really plus of brightness, but at high runtime cost).
3. The 6V and 12V little alkaline cells (MN21 and similar..) are good to use in 1AAA flashlights which you want to convert to leds. The disadvantage is they have only minimal capacity (as they are simple button cell stacks). And they are shorter than a AAA so you will have to use a larger spring or a spacer.
4. 3x AAAA (yes, 4 times A) are told to sometimes replace 1 AA cell, if there is a little air in the flashlight (they are a little bit larger). Havent tried this one for myself yet.
5. If you want to use NiCd rechargeable cells, you can buy them in very different sizes, usually parts of normal cells, for example 1/3AAA which means they have the same diameter, but only one third of the height of an AA cell. There are others to think about (1/2AA, 1/3AA etc....)
6. There are some Lithium cells in the form of standard batteries, but at higher voltage. You can buy Lithium-Thionylchloride cells (LiTCL) with 3.6V in forms of D, C, AA, and AAA cells. They have great capacity but are not able to deliver much current. Usually these cells are used for backup purposes for example for BIOSes in computers.
Another kind of Lithiumcells are the 3V types. They are used when it comes to higher current demand (in photos, flashers, surefires
, etc..). They only have 3V nominal voltage, but can deliver great current. They can be easily bought as CR123 or DL123 or other types for photocameras, but if you look a little bit you can get them in standard cases also (manufactured from SAFT-Alcatel in France for the military). One commercial LED flashlight runs on these cells: The HDS Action Light (
www.hdssystems.com if I remember right...).
Ok, if you want to match cells for own purposes, look at the page from the post above (data.energizer.com) where the measures are described. Or you can look at
www.rs-components.com which has a good variety of batteries also with specs.
Hope, I could help you a little bit...
