What's "big enough to be useful" for your needs? I do fine with a Bradford Guardian 3. That's a 3.5" blade carried horizontally on my belt. Very capable, comfortable, and concealable. Dozier makes smaller FBs with horizontal carry kydex sheaths. A couple of folks make concealable horizontal carry sheaths for Fiddleback Forge knives like the Hiking Buddy.
A couple of companies make front/back pocket sheaths that would fit the G3 or Hiking Buddy. Rick Lowe can make you a very nice and concealable angled belt sheath for about any FB you get.
"If there are no dogs in Heaven, then when I die I want to go where they went." - Will Rogers 1879-1935
RIP Riley, Molly, and Murphy... well done lads! AMOR AETERNUS
ISEK... and freedom of speech #12