Here's an answer posted in
http://goo.gl/Yb3nFj , pasted here in parts for your convenience.
This 手 originally conveyed a nuance of "handling," "doing," "dealing with", etc. and only later started to take on a more general meaning. So, speaking generally, the plain adjective might have many meanings, including gross physical ones, but the 手- version tends to refer to behavior or at least some implied "use case."
堅い = hard, solid, rigid, tough, firm, stubborn (of objects and people, physical and metaphorical) vs 手堅い = sure, firm (of behavior/attitude)
ぬるい = lukewarm, tepid, sluggish (of things and people, physical and metaphorical) vs てぬるい = lenient, non-harsh (of behavior/attitude)
狭い = narrow, restricted (of places/viewpoints/etc., physical and metaphorical) vs 手狭 = narrow (of places, when considered in the context of some [implied] behavior)
短い = short (physical, metaphorical) vs 手短か = brief, concise (of a text, etc.)
Another example: in Nishiwaki Junzaburo's translation of Shakespeare's 18th sonnet, the "rough windes" that "shake the darling buds of Maie" are 手荒い: "手荒い風は五月の蕾をふるわし..." The winds are active, they have behavior. (There may be some anthropomorphizing going on here too.)