An internal size and a method for sizing board, such as liquid packaging board, with a hydrophobising internal size containing oxetanon derivative or 2-oxetanon. The main part of the hydrocarbon chains R' and R'' of the oxetanon derivative or 2-oxetanon are derived from linear fatty acids, and .ltoreq.20 w-%, typically 2 to 15 w-%, most typically 5 to 12 w-% of the hydrocarbon chains are derived from branched and mainly saturated fatty acids.