Add this line to /system/build.prop (a few ROMs already have it, likely set to 240, in which case obviously just change the number):
200/210 is the setting I'm most comfortable with (sufficiently small but keyboard still usable).
Keep in mind that a few apps like Sony's launcher don't work well with DPI changes...
Motorola International 8700 (first phone sold in Italy to send SMS) [half bricked] > Acer ? [returned] > Siemens S35i [good] > Samsung SGH-? [stolen] > Nokia 3220 [mostly still OK] > Motorola RAZR [still OK] > iPhone 3G S [2 batteries worn out] > iPhone 4S [I gave up after iOS 7, still has 10 months of warranty] > Samsung Ace 2 i8160 [won at Samsung giveaway, still works fine, CM10.1] > Sony Xperia U [modified Sony 2.3.7]