Not knowing your conditions or intent it is hard to suggest specifics, but if you're planning to cut and move dirt, I would get the smallest blade that covers your tire tracks. My first suggestion/estimate would be one like this: 4-1/2 Foot Xtreme Duty Compact Box Blade by Everything Attachments The 4 foot unit is also a very viable option, and may work better, depending on your use.
The 4 1/2' width should easily cover the tracks, it is heavy enough to work well, and is very well built. It is also probably not too big to use for cutting ditches or other serious ground engaging work. If your use will be mostly smoothing out areas like driveways or horse arenas, you might be happier with a wider unit, but will compromise on digging ability.
Here is a little bit of my rationale for suggesting that particular unit:
The information I can find about your MT1601D says it is a little bit lighter (1300 pounds or so) than my Yanmar YM186D. With the compact tractors of this size and vintage, they virtually always run out of traction before power ever becomes an issue. My 186D weighs 1538lbs according to the manufacturer, but with the extra wheel weights, filled tires, side cultivators, and suitcase weights it easily weighs more than 2000 lbs: I have all 4 tires filled (at least 250 lbs) plus 175 lbs of cast iron wheel weights. The side cultivators are another hundred pounds, which takes the machine to a ton. I also have about 240 lbs of suitcase weights, and I'm another 245 lbs myself, so this tractor would weigh around 1000 pounds more than your machine with a 200 lb operator, a large difference.
I use either a 4 foot or a 4 1/2 foot box blade with my machine. The 4 foot I have brackets attached to add additional cast iron weight, because I want the blade to dig in and bite in my hard, compacted soil. I use the 4 1/2 foot for general purpose duties. Even with my dramatically heavier than original tractor, the 4 1/2 foot box blade will pull my tractor sideways if I have adjusted the side links to angle the implement to cut a drainage ditch. The 4 foot pulls much straighter. The 4 foot unit can be easily pulled when full of dirt, the 4 1/2 foot is just at the ragged edge of sometimes needing differential lock to be manageable when full.
On my similar sized Kumiai ST1440 (A rebadged Mitsubishi S373) I used a 42" box blade on that 1200 lb tractor, and it was fine, but a bit small. The weighted 48" blade was barely too big for that machine, which had no additional
ballast whatsoever. It wouldn't quite pull the 48" box when full.
Since your machine is between mine in weight, but has more power than either, I think the larger sized units are better suited to your purposes, since it is also likely that your conditions and use aren't as demanding as mine. If you have a loader, I would get the same width as your bucket or a smidgen narrower. I like to be able to dig a hole with my loader and drive down into it (For burying animal carcasses etc), and a rear implement the same width or wider than the bucket keeps that from being possible.