when buying tall-ish speaker stands for small speakers, should the dimensions of the top plate be greater or smaller than the dimensions of the speaker?
Im not sure how the speaker is secured to the stand, thats why im unsure.
Speakers are more often than not secured to the top of a stand with a material variously called BluTak, PlastiTak, PhotoTak or some brand name like that. Any office supply shop will have a material that is "pliable and reuseable" used for mounting materials such as documents. A very tiny amount about the size of a small pea will be sufficient in three spots on the underside of the speaker. Once the speaker is settled in position, if you need to remove the speaker, you accomplish this with a twisting motion on the speaker cabinet.
As to the stands, the top plate should be not much bigger, if at all, than the speaker's dimension.