What is the datatype of dna sample?
Assuming that you are looking at data used to describe the differences for a new individual, as opposed to a human reference genome build:

A fastq file is the typical format of data from a sequencer.

It would require a text field of some sort as they can be quite large, even for single reads.  If you had a specific sequencer in mind with very short read you might use a fixed length field for the data column, but its probably not worth it. 

After the data is processed, then the data is currently usually exchanged in the form of Variant Call Format files (a vcf file) which only documents differences versus a reference genome build.  This fits nicely into an SQL table with relatively small columns.