I carried traditional multiblade folders, bigger lockbacks, modern one handers, and fixed blades between 4"-6" as my EDC on the farm. They can all work, but naturally some are better than others. Also keep in mind if you're really using 'em, they will all get seriously beat up, and/or lost. I don't think I ever had a knife last more than a year or two.
The nice thing about a multiblade folder is you've always got a sharp fresh edge in reserve to get you through the day, plus multiple blade shapes for more specialized tasks. They aren't quite as handy to use (diggin' em out of the bottom of your pocket when your hands are completelly covered in grease, then needing both hands to open).
A one handed folder of generous blade size has the advantage of locking; the point of my farm knives got used a lot. Digging to check seeding depth, poking all manner of things, etc. It is handier than either a traditional folder or fixed blade. Pocket clip keeps it easy to access & deploy with one hand, and you can close it & put it back in the pocket blindly. A fixed blade needs attention when resheathing so you don't stick yourself, and this is especially unhandy in winter when the sheath is usually under a heavy coat or something. For a tool that may get pulled out 10-50 times a day, this is a consideration.
Fixed blades can have the advantage of being more rugged, so you can beat on 'em, and use the point without fear of any lock failure. I liked my general carry piece to have a blade at least 4"; 6" was the upper end of what never left my belt. Not a big deal either way. The tasks are so varied there's no way you can say a similar knife with an inch more/less blade will be better- it's up to your preferance as much as anything. It needs a combination of stout & strong proportions, coupled with an edge thin enough for good cutting ability. I'd also prefer a real guard & some kind of pommel or buttcap to protect the handle materal. I find a narrow point useful. A false edge is good for scraping, so you can spare the main edge. A tough impact resistant steel is a good idea. The scabbard needs to be cut resistant & easy to insert the knife back in it without fumbling.
I don't think any of these can take the place of big blades, such as machetes, corn/fodder knives, brush axes, bills, etc. I carried them as well when I expected to need them. And these can't take the place of chainsaws & loader tractors.
You may as well get yourself a decent pair of pliers while you're at it. They'll be used as much as the knife. Some guys carried the multi tools with pliers built in, but I just never got into them. I can see where a multi tool would pair up very well with a fixed blade knife.