user: Question: Which ERP W is the highest one that has a Call sign of W255BI?
Columns: ['Call sign', 'Frequency MHz', 'City of license', 'ERP W', 'Class', 'FCC info']
Types: ['text', 'real', 'text', 'real', 'text', 'text']
Sample row: ['W264BG', 100.7, 'Benedicta, Maine', 19.0, 'D', 'FCC']
SQL:

assistant: SELECT MAX("ERP W") FROM "Table" WHERE "Call sign" = 'W255BI';