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You can't change the length of a frame, but if you need some more for a single frame then you can use the frame after and do a D00 command to make it shorter.
i suppose you could have an effect command which is the antithesis of D00 - where you specify in xx the number of extra rows you need for that frame. you can do more or less the same thing using D00 with a new frame though. different means, same end.
i do like the idea of time signatures though -- i'm thinking along the lines of having time signature presets (ie. user chooses a time signature, and the tracker understands that time signature as being XX rows per frame.) the only problem is there's no standard as to how notes are spaced on the grid. so 64 rows can be 2 bars of 4/4, 4 bars of 4/4, or even 1 bar of 4/4 if you treat it as 16 rows per beat.
it'd be a cool thing to consider though. it'd cut out the manual mathwork involved in setting up the right amount of rows for exotic time signatures!