The Heemskerck was built in 1638, the same year as the small ships, the Utrecht, and Overijssel, both carrying 22 guns. The latter two ships are usually credited with having dimensions of 100ft x 23ft x 8ft. As I have written elsewhere, these ships were actually larger, when measured in Amsterdam feet. The published dimensions were in Maas feet, which were divided into 12 inches. Amsterdam feet were 283mm and divided into 11 inches. I believe Maas feet to be 308.7mm, which is roughly equivalent to multiplying 208 x 12/11.
My note about the Utrecht and Overijssel is at Kentishknock.com. In any case, the Heemskerck was very similar in size to these two ships. Possibly, they were all the same dimensions, and the dimensions in Maas feet don't quite equate to the dimensions in Amsterdam feet. I theorize that might be the case. The Utrecht and Overijssel were probably about 100 lasts, as well.