Currently, we always show the 5x5 item slots, even when only a few search results are returned. This can be quite intrusive and awkward. Ideally we would be able to select a smaller texture depending on how many results are displayed.

JEI's NineSlice drawing thingy code may be pretty good for this purpose, you can use the one existing texture and resize it in code:
https://github.com/mezz/JustEnoughItems/blob/7571d601d8d74170d7384952b07e564d0f7c11a3/src/main/java/mezz/jei/gui/elements/DrawableNineSliceTexture.java#L17-L21