See if this one will work for you.It's really small.
ZYX Artisan, if you are looking for something at the higher quality end of the spectrum. Its v small (6"W x 2 1/2"H x 7 1/2"D) runs on a battery which is charged by a wall-wart and really has no controls beyond on/off and mm/mc. Given the very low power demand, I would leave it plugged in and turned on all the time; and of course you would not need to adjust the mm/mc switch unless you changed carts.
Lower cost but still very good option would be Lehman Black Cube. Its about 4.5" x 4.25" x 1.75".
A final choice would be Benz-Lukaschek PP-1. which as I recall is about the size of a good-size universal remote; maybe about 3" x 8" x 1". I owned all three but used them with different carts. I liked the Artisan the best and the Benz the least, but both Benz and Black Cube now have newer versions on the market. And the Artisan is available at a heavily discounted price from Sorasound, which, unless there are special synergy issues, kind of makes the Benz superfluous.
Try the Musical Surroundings Phenomena. It costs $600 and has huge adjustability to suit nearly any cartridge. I have one paired to a Benz Micro ACE low output and it is magnificent. It also easily meets your size requirements and is solid-state, always-on. So, you plug it in and forget about it. It will be perfectly happy behind your pre-amp.