Dont Worry only in rare cases have I ever saw a Lima Shovelnose Pass 18"...The largest one I have ever seen was one that was around 21"...The fish was massive and had to have been atleast 10 years old....Lima's grow rather slow at most maybe 1/2" to 1" per month when powerfeeding, after they reach around 8-10" they slow down dramatically maybe at the most 1/2" per month from that point on. The reason why you would need a large tank is because Lima's do have their bouts of skiddishness and will run around hitting their sensitive noses on the edges/ornaments in your tank. I have a 125 gallon and my lima is around 6" I have had it for about 3 months but I have owned other Lima's for 5-6 years....the biggest one never reaching over 11" so the more common 14" though 18" is what i would expect, within around 4-6 years time. Dont worry even then as it grows it loses alot of its skiddishness so dont worry!
Buy 3 or 4 of them and a pleco and enjoy!
WARNING: Has tendencies to eat anything that is fast moving! Do not trust around smaller fish! Anything that could/should/would be food is considered edible to these cats. Bright flashes of light/fast movements/tank tapping/ water changes and other stress factors may trigger a reaction. This fish does shed its slimecoat several times a year!