The DITA table element is based on the OASIS model, which does not really provide a way to specify overall table size; all you get is @pgwide, which for XHTML means nothing at all. The colspec element has @colwidth, which “describes the column width”. For the simpletable element, DITA provides only @relcolwidth, with its odd n* method of setting percentages. Your best bet is to use CSS syntax and semantics for column “width”.
By default, table columns are adaptively sized in HTML output. You can specify a different sizing method in the configuration file. You can also override the default sizing method for particular tables.
§33.4.9.1 Specifying a method for determining table column widths
§33.4.9.2 Overriding the default table column sizing method
§33.4.9.3 Maintaining the width of table columns via relative sizing