I usually tell you to trim a panel to a specific measurement. If there are pieced blocks in the border to add around the panel, the panel has to be a certain size to fit it all together.
Zig or Zag is one that requires exact measurements. The pieced blocks in the border add up to a specific size, so the panel plus the first border, must be a specific size for the pieced blocks to fit. Chain of Geese, Diamonds and Bowties, Diamond Daze, and Around the Block also need to be exact measurements. The panel length and/or width could be less and you could add a border strip(s) to make it bigger to fit, but if your panel is larger, then it needs to be trimmed.
On the other hand, Goose Chase, Fly Away, Borderline, Whirlwind, Lanterns, and Deadends all have squares or rectangles that can be adjusted to fit a longer panel. For Whirlwind, I would suggest adjusting the size of more than one of the squares to keep a uniform length.
One of the testers of Deadends said she had a panel that she didn't want to trim down to the size in the pattern. That's a very simple change, make the Rail Fence blocks longer.
For example, if the instructions say to trim to 39", but your panel is 42", you would add 3" to the Rail Fence block.
Keep in mind that if you change the trimmed size of the panel and the size of the pieced border blocks it will also change the size of any border after that, you'll need to add onto those the same amount you added to the panel. The backing and binding also need to change. That might mean you need more yardage than specified on the pattern.
I hope this gives you some ideas for dealing with various size panels.
