PRIMER_PRODUCT_SIZE_RANGE (size range list)
Product Size Ranges
The associated values specify the lengths of the product that the user wants the primers to create, and is a space separated list of elements of the form
where an - pair is a legal range of lengths for the product. For example, if one wants PCR products to be between 100 to 150 bases (inclusive) then one would set this parameter to 100-150. If one desires PCR products in either the range from 100 to 150 bases or in the range from 200 to 250 bases then one would set this parameter to 100-150 200-250.
Primer3 favors product-size ranges to the left side of the parameter string. Primer3 will return legal primers pairs in the first range regardless the value of the objective function for pairs in subsequent ranges. Only if there are an insufficient number of primers in the first range will Primer3 return primers in a subsequent range.
For those with primarily a computational background, the PCR product size is the size (in base pairs) of the DNA fragment that would be produced by the PCR reaction on the given sequence template. This would, of course, include the primers themselves.