(I assume you mean GW not MW, because MW is very little for a proper power plant). Solar panels produce about 200Wp/sqm. In Germany they average to about 12% of peak production over a year, so say 22W/sqm averaged. 6GW continuous is then about 51k football fields[1]. I assume that the US has lots of land that is much sunnier than Germany, so you can probably get away with fewer football fields.

[1] https://www.wolframalpha.com/input?i=%286GW%2F%2820W+per+squ...