A method of sizing a repeat region of a nucleic acid sample, comprising: i. amplifying the repeat region; ii. performing high resolution fragment analysis that is capillary electrophoresis; iii. obtaining a ladder of amplification products comprised in one or more channels in a file; and iv. generating an internal sizing standard using the ladder of amplification products by: identifying a repeat profile in the channel of said file corresponding to the ladder of amplification products; iteratively generating a set of repeat peak locations, the iterative generation comprising: determining a predicted peak location in the repeat profile using a previous peak location and an interval value, wherein the interval value is the average of the differences in location between the actual peak and the previous peak for two or more previous iterations, identifying a peak location within a window including the predicted peak location, and adding the identified peak location to the set of repeat peak locations when a signal intensity at the identified peak location satisfies an amplitude criterion; and estimating a linear relationship between repeat peak location and repeat fragment size using the set of repeat peak locations and a set of corresponding fragment sizes; and v. sizing the repeat region using the internal sizing standard; wherein the internal standard is used without any external standards. i. amplifying the repeat region; ii. performing high resolution fragment analysis that is capillary electrophoresis; iii. obtaining a ladder of amplification products comprised in one or more channels in a file; and iv. generating an internal sizing standard using the ladder of amplification products by: identifying a repeat profile in the channel of said file corresponding to the ladder of amplification products; iteratively generating a set of repeat peak locations, the iterative generation comprising: determining a predicted peak location in the repeat profile using a previous peak location and an interval value, wherein the interval value is the average of the differences in location between the actual peak and the previous peak for two or more previous iterations, identifying a peak location within a window including the predicted peak location, and adding the identified peak location to the set of repeat peak locations when a signal intensity at the identified peak location satisfies an amplitude criterion; and estimating a linear relationship between repeat peak location and repeat fragment size using the set of repeat peak locations and a set of corresponding fragment sizes; and identifying a repeat profile in the channel of said file corresponding to the ladder of amplification products; iteratively generating a set of repeat peak locations, the iterative generation comprising: determining a predicted peak location in the repeat profile using a previous peak location and an interval value, wherein the interval value is the average of the differences in location between the actual peak and the previous peak for two or more previous iterations, identifying a peak location within a window including the predicted peak location, and adding the identified peak location to the set of repeat peak locations when a signal intensity at the identified peak location satisfies an amplitude criterion; and estimating a linear relationship between repeat peak location and repeat fragment size using the set of repeat peak locations and a set of corresponding fragment sizes; and determining a predicted peak location in the repeat profile using a previous peak location and an interval value, wherein the interval value is the average of the differences in location between the actual peak and the previous peak for two or more previous iterations, identifying a peak location within a window including the predicted peak location, and adding the identified peak location to the set of repeat peak locations when a signal intensity at the identified peak location satisfies an amplitude criterion; and estimating a linear relationship between repeat peak location and repeat fragment size using the set of repeat peak locations and a set of corresponding fragment sizes; and v. sizing the repeat region using the internal sizing standard;