I did some calculation and found out, that my Living End deck (modern) might be better of with 66 cards maindeck. SB still contains of 15 cards. Depending on the match up, I'd like to cut my deck for game two down to the normal 60. The SB than would contain 15+6 = 21 cards. Is this legal?
In constructed play your sideboard can not contain more than 15 cards at any time. The relevant rule here is rule 100.4a from the comprehensive rules, for constructed play:
100.4. Each player may also have a sideboard, which is a group of additional cards the player may use to modify his or her deck between games of a match.
100.4a In constructed play, a sideboard may contain no more than fifteen cards. The four-card limit (see rule 100.2a) applies to the combined deck and sideboard.
No specific circumstances are mentioned so it applies at all times. This rule was updated with the M14 core set. Matt Tabak explained it as follows in the M14 rules preview article (emphasis mine):
With the new rule, your main deck is still sixty or more cards but your sideboard is now up to fifteen cards. Additionally, you are not required to swap cards between your main deck and sideboard on a one-for-one basis. For Games 2 and 3 (and so on), as long as your main deck is sixty or more cards and your sideboard is no more than fifteen cards, you're good.