According to a method of determining a size of a sample polynucleotide, a sample polynucleotide is subjected to electrophoresis in the presence of a fluorescent compound having a first fluorescence spectrum. Detection of light of the first fluorescence spectrum is indicative of the presence of the sample polynucleotide. One or more size standards are also subjected to electrophoresis, optionally in the presence of the sample polynucleotide. If more than one size standard is used, the different size standards typically have different mobilities. The size standards are generally essentially or completely free of polynucleotides. Migration coordinates, e.g., migration times, of the sample polynucleotide and size standard(s) are determined. A size of the sample polynucleotide can be determined using the migration coordinate of the sample polynucleotide and the migration coordinate(s) of the size standard(s).