Equipment should be sized to the job done. The major factors are HOW GALLONS PRE THOUSAND are you applying. There is a relationship of Quality or response of the fertilizer to the volume of water applied.
The more water the better the response and LESS PROBLEM with operator error.
The more water used means Transportation issues.
Bed space issue can be a problem when you are using one truck as a utility vehicle. I suggest you might want to set up a trailer spray rig that is only pulled on the few days you are spraying. As a one man band I have 2 spray trucks and two trailers set up with different products. The trailers are used only once in a while for special jobs in my case.
Back to Gallon per Thousand applied. No two people are going to agree on how liquid fertilizer should be applied. I do 5 gallons per thousand some go 10 and even more. But most go 2 to 3 gallons per thousand so they can cover more ground per tank. Some use as little as 1/2 gallon. I am not going to talk WHY.
What Tank size??? I like 300 gallon tanks but I am not you. Remember you can alway mix a 1/4 or 1/2 of a Big tank but you can't mix 2 times as much in a small tank.
it is one thing to do a off label applications. It is total stupidity to post it on the world wide web and expect people to approve.
You can lead a Donkey to water but you can't make the Jackass Drink