With most over the counter and on the top shelf Bed Linens choices we face the 1 size for Fitted sheets that will be adjusted over time for the depth of a mattress.  Adjustments come from the dryer if the sheet is all cotton when it is left too long at high temps as it will shrink.  If your mattress is around a basic 11 to 13 inches deep, your best choice should be 16 to 17 inches to allow for this shrinkage.  Besides the mattress depth you should include the mattress pad of 1 inches or less (unless yours is a wool or down mattress pad).

Fitted Sheets can be custom sized to fit your mattress depth.  Check with the Linen Store if you prefer your Fitted sheet to be totally a different size than offered on the shelf.  Most times there is not an upcharge if made smaller yet there would be a wait time.  You should ask before assuming no extra charge.  Fabrics may vary with cost per yardage so any adjustments would be an extra fee.  And, do allow for that extra shrink factor from the dryer.

Although Linen loves to be laundered and in the dryer with mild heat, you should not allow this too much as over time your sheets will size down in the depth and in the length.

A size chart:
Mattress: 11 inches to 13 inches look at the 16 to 17 inches pockets
Mattress: 15 inches to 17 inches look at the custom size of 18 to 20 inches pockets.
  at this point you may have a seamed edge if custom sized King is needed.
Mattress: 7 to 9 inches look at the 12 inches to 13 inches pockets.

Be careful not so size your fitted sheet too small to be just popping up each use as it will not adjust to the mattress to be in place at all times.  Another good measure is to allow a 3 inches tuck under the mattress to be secured.  In any case, it is best to ask for help.