5x7 film can be difficult to obtain these days. The suggestion to trade for an equal amount of 4x5 film is good advice, for the following reasons:
1. When you trim down film you subject it to dust and potential of scratching the film.
2. While you might be able to get close, there is a possibility that you don't cut it to precise size, meaning they'll fit your film holders poorly.
If you get equivalent film in 4x5 size, you can rest assured it will be cut to exactly the correct dimension, and you stand a better chance of having film that is essentially scratch free, and you save yourself the work of trying to cut the film in complete darkness.
Just my two cents worth.