Depends on if the OP means this should be event-driven/dynamic or not. The "when" seems to imply that it should be. When .widget becomes larger than 300px (screen resize/flip, for instance), then the font-size should become larger.
If that is OP's intention, then I feel @when is more appropriate than @if
If that is OP's intention, then I feel @when is more appropriate than @if