If you're trying to change an energy value which maxes out at 100, choose 1 byte (which has a range of 0 to 255).
If you're trying to change HP and it maxes out in the game at 9999, choose 2 bytes (which has a range of 0 to 65536), etc.
It usually is inadvisable to use larger memory sizes than required, as it may have undesirable effects on your game.
The cheat system described here is referred to as 'raw' or 'internal' cheats.
The cheat searching menu can be found in the toolbar under : Emulation | Cheats | Search.
The initial cheat window has the following options: Select the memory addresses' size for the value you are searching for.
Choose the smallest range applicable to the value you're trying to change.
This manual explains the 'raw' or 'internal' cheat system in desmume.
The 'raw' or 'internal' cheats include cheat-finding facilities where you may manually search for and alter the memory positions which work to your advantage.
This article will provide a rundown of the cheat searching menu and functions, followed by a pair of examples.
Please note that, in post-0.9.2 versions, there is also the support for Action Replay cheat codes.
A signed memory refers to a memory that can include both positive or negative numbers.
So if the value you're searching for can be less than zero, it is likely to be a signed memory type.
Do note that when choosing a memory as signed, the range is halved in both directions.