It will depend upon your state laws, the terms of your insurance policy and the guidelines of your insurance provider if a crack on a windshield must be a certain size before the insurance company will accept a claim for the replacement of a windshield. If the crack on the windshield an insurer believes is repairable they may since try to repair to your windshield.
In many states insurance companies can indeed decide to repair the windshield crack or chip or replace the whole windshield. If the windshield crack can be repaired, which is usually done with a specially formulated resin being injected into the damaged area and then the resin cured, then an insurance company may choose to do so since it is a cheaper way in which to take care of a small windshield problem.
A good repair job to the windshield should restore its structural integrity and restore clarity to the area so your vision is not impaired around the area which the crack was. If after the repair your vision is still impair in the area of the crack then a glass company will normally decide that the glass should be replaced.
Repair jobs are cheaper than replacements and thus one reason they are used by insurance companies for small cracks or chips. Also by repairing instead of replacing the windshield the integrity of the original factory seal is not affected since the glass is not taken out and you keep your original manufacturer quality glass in your vehicle.
To see what state laws are regarding insurance claims and your front windshield contact your state's insurance regulatory body. To find out the guidelines of your particular insurance you can read through the terms of your comprehensive coverage to see if it speaks about repair versus replacement of glass on your vehicle. You also can speak to your insurance agent about what the internal guidelines of your specific insurance carrier are regarding windshield cracks and when they feel it is a cause for the whole windshield to be replaced instead of repaired.
There is no universal size of a crack that all insurer use to determine if a windshield should be replaced. For instance some may say that a crack the size of a quarter and that is spidering out is cause for replacement while another insurer may say the chip or crack must be the size of a dollar. The placement of the crack can make a difference as well as the size. If the crack is obstructing the driver's vision of the roadway and a repair would still leave the driver's view impaired then a replacement of the windshield may be required by the state and insurance provider instead of a repair.
A glass repair / replacement shop will be able to tell you if your windshield can be repaired or if the type of crack you have needs the whole glass to be replaced. If it can be repaired then you may want to see if it is worth making a claim or if the cost of the repair is less than your deductible. Also sometimes an insurance company will waive the deductible if your repair your glass instead of replace it so if you choose to go through your insurance to get the windshield crack repaired you may ask about having the deductible waived.