As a general rule, you want tongue length, measured from the front of the box to the center of the coupler, to be at least half the width of the floor of the trailer.
I just now measured 4 of my trailer's tongues, from the front of the box of the trailer to the center of the coupler. The trailer length and width listed below for all 4 trailers is the box only, not including tongue or rear bumper/spare tire or fenders.
5'x10' utility, single axle "motorcycle trailer" = 35"
6.5"x16' utility flatbed, tandem axle = 36"
7'x14' enclosed cargo, tandem axle= 43"
8'x20' TT, tandem axle = 46"
With any of those, you cannot turn sharp enough when going forward to have trailer to tow vehicle contact. But put that puppy in reverse and you can have a full jackknife in a heartbeat.
So for backing the trailer without any problem to a full 90° jackknife, you want the tongue length to be at least half the width of the trailer. If your cargo trailer is an 8.5' (or 102") wide "race trailer", then you'd want the tongue to be at least 51" from the front of the box to the center of the coupler. The more common 8' wide cargo trailer, then 48" tongue length is fine. Note that my 7' wide cargo trailer needs at least 42" tongue length, and it has 43".