I cut down my bike boxes. The magic numbers are 54” x 29” x 9”. I just shipped a bike in that cut down box for $92.87. If I had left it the original size (55” x 31” x 9” one inch longer and two inches higher) it would have cost $178.32 to ship.
Assuming people here are using a third party online subcontracting service like Bikeflights or Pirate Ship to get the best rates and print their own labels and with the actual shipping duties being handled by UPS either by pick up or drop off. Obviously retail or counter rates are cost prohibitive.
To expand on Lester's good information the largest size a carton can be for UPS without accruing additional charges must be exactly at or less than 130" using this formula...
Length plus girth or Length plus 2 x Height plus 2 x Width (L + 2H + 2W=130)
Lester's example: 54" + ( 2 x 29"=58") + (2 x 9"=18")=130"
Remember you can use other dimensions so long as they are less than or equal to 130".
For instance 56" x 28" x 9" etc..
It's critical your box must truly be exactly 130" or less or you will be charged the oversized rates. I use Pirate Ship and it's happened a couple times where a week or so after the package has gone out I get a notification of an adjustment of $60 or $70 more because the package was borderline too big. You do have a recourse and you can dispute this and I've reversed it several times by stating that I have photo documentation of the dimensions etc., but if you do not dispute you will be charged!

So when cutting down a box you really should shoot for your dimensions being slightly smaller than the 130". Oftentimes I will reduce my length and height measurement by 1/2" just to be safe.
And lastly and this is important don't make your carton look like you stuffed a cow

in it as this will raise the alarm at UPS center!
NVT