What scale should a detected SIFT point be extracted at?
If I understand correctly what you are asking --
In general, the feature is found at the same scale as the SIFT detector says, but
in David Lowe's SIFT, the image is pre-smoothed with sigma: 0.5, so, you need to "subtract" this amount of smoothing from the sigma, so the "real" scale could be: 

sqrt(sigma^2 - 0.5^2)

where sigma is the scale the feature was found at.
you have an excellent implementation of SIFT detector and descriptor at: www.vlfeat.org