I've been working through Quilter's Academy Volume 1 and they lay out how to calculate fabric needs and along the way I picked up how to recalculate at least for simple blocks. I haven't tried to re-size a complex block yet.
Basically break it down into component parts and also figure out what your basic block grid is - is it 3x3; 4x4; 5x5 - as this will make a difference in how large a block ends up.
The Celestial Circles quilt on my blog is enlarged to accommodate the suns. Ideally, it would have been a half inch smaller for each block to trim a little closer into the suns, but that put me at a number not divisible easily by 3 which is the basis for the grid.
Cheers, K