I want to watercool my HDD and then enclose it in a sound proofing material so i'm looking for a relatively small cooler so that ther will be room for the soundproofing material.
Any suggestions?

I have to ask - why do you want to watercool your drive? If it's for looks/kudos then fair enough, but HDDs don't need to be watercooled in regard to running temperature. They need very little airflow to stay in a safe operating range - the kind of airflow you get from a watercooling radiator anyway.

. . I have & use both the Alphacool 'Silentstar' device [2x copper waterblocks, screwed to each side of one or two HDDs & connected together via tube, all assembled in a foam-lined stainless-steel box]; & a pair of similar dual-HDD copper waterblocks from EDVExtreme.

The 'Silentstar' is also available in a single-HDD model.

Both work very well indeed for cooling, & the Alphacool device [which is now also & in turn enclosed in a custom-made sheet lead box] makes a pair of Raptors genuinely quiet, even by silentpcreview standards.