Just edit /usr/share/themes/{yourtheme}/gtk-2.0/gtkrc file (as root) and find out these lines:

Replace the 26 with 38 and 22 with 42.

EDIT: On some themes, this may result in duplicate scrollbars . Thats not very nice...

Heres my diff that I took from regular 14pt echo theme and wide scrollbar version (the above numeric changes were there as well):
14pt echo: style "hildon-im-thumb-scv-font"

Wide scrollbar: style "hildon-im-thumb-scv" = "osso-logical-colors"

14pt echo: widget "*osso-im-scv-renderer" style "hildon-im-thumb-scv-fon

Wide scrollbar: widget "*osso-im-scv-renderer" style "hildon-im-thumb-scv"

So just use the wide scrollbar thingies if you get the duplicate scrollbars, I guess.