Radar images are improving as this video of 1998 WT24 shows. Only 400 meters across and 4.3 million km / 2.7 million miles away, this is great resolution.
We have a VERY close encounter coming up today. At 12:07 today Asteroid 2105 YB is going to pass just 0.2 LD from the Earth. Thats only 76,880 km / 47,770 miles, very close indeed. Note there is some uncertainty of the time of closest passage, it’s possibly up to 1.7 hours off. 2105 YB is 10 meters in diameter and was only discovered a few days ago so the number of observations are limited so far and as you can imagine these things are terrifically hard to see.
More observations will improve the accuracy of predicted future positions. 2105 YB might make a good choice for a radar target as any asteroid this close could (not saying it does) pose a potential risk.