Better off to err on the "high end" since the bars are rated to handle loads below their top rating, but not above. For example, here are the specs of a Reese WD system with 1200 lb trunnion bars. Note in the specs it says for tongue weights of 600-1200 lbs. I`m not saying you need 1200 lb bars, these specs are just an example.
Reese Weight Distribution System w Shank - Trunnion Bar - 12,000 lbs GTW, 1,200 lbs TW
Tongue weight: 600 lbs - 1,200 lbs
Gross towing weight: 12,000 lbs
Fits: 2" x 2" trailer hitch receivers rated for use with weight-distribution systems
Lift-bracket positioning: mount about 30" back from hitch ball
Requires approximately 5-1/2" of space along trailer frame
Optional mounts (sold separately) that take up less space on a trailer frame are available
Will not fit trailer frames wider than 2-1/2"
Shank length: 14" from center of hitch pin hole to center of ball hole
Total height adjustment along shank: 5-1/4"
Maximum rise: 6-1/2" from top of hitch receiver opening
Maximum drop: 1-1/4" from top of hitch receiver opening