simG wrote:
That what I did and the results aren't the same.

What degree of "not the same"? Like totally wrong or off by a bit in the decimal places? Maybe some example would help. Also, if you are familiar with syntax, could you lay out your regression syntax and the calculation syntax? And if not too much trouble, can you also show the regression output?

Another possible problem is maybe you have used the "Standardized beta" in the regression coefficient output, while the actual ones we should use are the coefficient listed under the first column called "B".

simG wrote:
as for criteria for generalization, I know there are a lot, I'm asking about sample size specifically.

Then that'd depend on your power, suggested effect size, and variation among your sample and population. If your slope is steep and the variation is small, then a small sample can give unbiased estimates. If your slope is shallow and the variation is big, then a bigger sample size is needed. This is not a question that can be answered by just the sample size alone.