East Grand Forks, MN has a channel catfish tourney every summer, and a lot of people turn out for it. Since Fargo is only about 60 miles farther south on the Red, I would imagine that cats make it there as well. What their size is, I don't know, but I suspect that the farther north you go, the bigger the cats get. The prime fishing area for huge cats is Lockport, north of Winnipeg (which is only 1 hour north of the US border, or about 3 hrs north of Fargo). Travel across the border is no problem, just have proper ID and your boat and car registration papers. Leave any handguns at home.
I'm Canadian and travel regularly to Fargo and Grand Forks to shop on any given weekend, and the drive is easy from Winnipeg. I was there last week buying 6/0 Gamakatsu circle hooks, because cat fishing is so popular here in Winnipeg that the stores are always sold out.
Here's a site from Lockport, where the US plates often outnumber Canadian plates:
http://www.catsonthered.net/Wherever you decide to fish, have fun, the big guys really put up a fight!