This Instructable describes resizing an IKEA Hoppvals Cellular Blind. The Ikea site states that it is not adjustable by cutting, I only found this out after I bought the blinds (admittedly on the cheap, £11 heh, heh).
I had the previous Bruddans Blinds from Ikea and they could be cut by about 100mm, but as they say, never assume.
Obviously this process is going to invalidate your warranty, but these blinds cost £25 ($34, 28 Euro) and the nearest I could find for fitted blinds came to £84 (the most expensive was nearly £400, I might do this for a living).
The Hoppvals blind has the spring mechanism very close to the end of the top frame leaving no room for adjustment. I had already bought the blinds so what to do...
Step 1: Top Frame
The spring assembly which lifts the blind is about 30mm from the end, I don't think I could cut that and still be usable. The other end is much the same.
First thing: take off the end caps and put them somewhere safe.
Step 2: The Spring Assembly
The Spring assembly is held in place by tension against a small bulge in the top frame
I pulled the Spring Assembly out to the end and let it rest, the whole thing is under a lot of tension so be careful.
Step 3: Bending the Front Tab
Take your pliers and bend the front tab on the Spring Assembly so that it is flat and will pass over the bulge in the top frame. The rear tab will catch instead moving the Assembly 80mm along the frame.
Step 4: Reseating the Assembly
The Spring Assembly is 80mm closer and this has released quite a bit of tension. Before putting the Assembly back in tie up 80mm (2x40mm) of the cord near to the central pulley block.
Remembering the tension in the spring, pull the Assembly to the end of the frame and carefully put it back in the top frame. DON'T LET GO! It will pull quite strongly. Slowly move it back until the rear tab stops it against the bulge in the frame.
Voila! I now have 80mm (3"1/8) to play with.
Step 5: Cutting Time
Line up the top and bottom frames with the blind material at the cutting end.
The blind should be shorter, leave the difference at the uncut end. Mark off how much you want to cut.
Pull the frame back. cut the material with a scalpel, bread knife or hacksaw.
Take some card (I use the packaging) and bind up the whole blind with tape to protect the finish whilst sawing.
The frames are aluminium, a junior hacksaw would do. I couldn't find mine so I used a jigsaw with a bimetal blade, a bit of overkill.
Step 6: ...Add Tension
Put on the end caps and try out the tension on the blind.
You should be able to easy pull down the blind, have it stay in position, and push back up again with a finger, again staying in the closed position.
I also cut the support brackets to fit the blinds very close to the window.
And now I have to fit them. Enjoy.