You can, if you are communicating with people in the US. The US is the last major nation to somehow fail to convert to the metric system. When interacting with people in the international community it would be better to use square kilometers. This will allow a quicker and more accurate comparison with land areas as they are reported across the globe.
A good estimate of the distance between California and Wales would be about 5,300 miles or 8,420 kilometers. However, California is a big state, covering 163,696 square miles (423,970 square kilometers), so the exact distance would have to calculated from your exact departure point.
That would be Rhode Island with 1,545 square miles. Washington D.C., although not a state, but a federally managed district, is 68.25 square miles.
The closest in size would be Texas with 268,580.82 square miles.
That would be Texas with 268,580 square miles of total area.
None. The closest would be California with 163,695.57 square miles.
None. The closest would be Rhode Island with 1,545.05 square miles.
At 147,046 square miles Montana is closet to Germany's 137,847 square miles. Next in size would be New Mexico at 121,593 square miles.
Mississippi is a state that is approximately 46,000 square miles in size.
The state of Washington is 71,303 square miles.
It is the 35th largest US State containing 42,774 square miles.
The state with an area closest to 4,244 square miles is Connecticut at 5,544 square miles.
Hawaii is the state with an area of about 13,000 square miles.