Zach - to expand a little on Nigel's responses.
For a given frequency, changes in the length of an antenna do affect the feedpoint characteristics. For simplicity we often think in terms of 1/4 wavelength, 1/2 wavelength, 5/8 wavelength. A half-wave dipole for 21 mHz would be about 22 ft long and is assumed to have a feedpoint impedance of 50 ohms (it doesn't but to keep it simple we'll stick w/50 ohms). If you made a dipole somewhat longer - say 30 ft - it might not work at 21 mHz. Let's keep going.
Multiples of these can be considered if space allows or your needs dictate. A good example of this - it's common for amateur radio operators to cut a dipole antenna for 1/2 wavelength at 7 mHz (40 meter band) and use it for both 7 mHz and 21 mHz - at both 7 mHz and 21 mHz the antenna can look like about 50 ohms at the feedpoint. At first the 67 ft long antenna might look like it's way too long for 21 mHz but it's not.
A loading coil can help to make a short antenna look longer. Coil characteristics depend on placement of the coil. While too complicated for me to explain here, there are numerous methods worked out in antenna and amateur handbooks or publications. Construction is fairly straightforward and good results can be achieved by most hobbyists. Take a look on the ARRL website or similar radio amateur websites.
For what it's worth a capacitor can make an antenna that is slightly long "look" a little shorter electrically. Heath used to make a motorized variable capacitor that could be placed across the feedpoint of a slightly long dipole. It would be adjusted to tune the antenna more precisely.
Good luck.