Generally speaking, I would say best practice is to create assets at the resolution you are going to use them in. There will always be some sort of loss or interpolation going on when you resize anything larger or smaller.
The second best method I would go for would be to have your original art at a much higher resolution and downsize to whatever is required. You might have some wasted effort or loss of detail, but at least it won't be blurry.
This applies to assets made in Construct or any other program as well.