Your title and text conflict. Do you want to fill a formula down a column (many rows) or across a row (many columns)?
Also, it is very inefficient to copy a formula down an entire column (65,000+ rows). If you are trying to copy the formula down a column, and if there is data going all the way down in the column to the left or right of the formula column, simply hover your mouse over the lower right corner of the first formula cell until it turns to a cross (plus-sign) then double-click.
Same cannot be done with going across columns in a single row, though.