If you're going to use those Dunlop MX71
tires, I'd go with sizes 80/100-21 and 110/90-18.
I don't use Dunlop
MX71s myself, but I really like the Bridgestone
competitor to those, the Bridgestone
M23 front and M22 rear.
These are also hard terrain tires, and for that, they are excellent.
I've used these tires on my off-road and dual-purpose bikes for at least 20 years (they came out in 1981, believe it or not, and are still produced).
The sizes I use with the Bridgestones are 80/100-21 and 100/100-18 (which is actually the proper-sized rear tire for your WR-250R, in all honesty, as the MX71 choices are both a bit bigger).
These days, I use them on my 2009 WR-250F.
They are full-on motocross/off-road tires, too, so they're not DOT aproved.
The MX71 or the Bridgestones will hold together on the street, but they won't last long if you ride some substantial milage with them on the street.
I wouldn't expect them to remain nice and new for many rides if you have to ride the pavement for 30 minutes to get to your riding spot.
In fact, after one hour on the street, you'll be noticing the wear.
That's to be expected with blocks of rubber sticking up an inch or so with nice, sharp edges when new.
DOT tires, like the stock tires that come on a WR-250R, are actually quite safe to use on the street because that's really their main mission in life.
Their off-road ability is basically dirt roads and easy stuff like that.
If you want to ride it like a dirt bike, you need some knobbies, for sure.
Motocross knobbies are not as safe on the street because they have less rubber touching the road, and the rubber compound won't last that long on the road.