Why are different lengths of nucleotides taken for structure prediction from an miRNA match area after BLAST analysis?
There is no hard and fast rule for how many nucleotides you have to select. If your miRNA is ~20nt and you take 70nt from both sides (average length of hairpins is ~84nt in humans), your total length will be 160nt. The point of taking $\pm70nt$ is that you don't know if your read is -3p or -5p. So take 70nt from both sides. 

The maximum length of a human miRNA hairpin is 180nt for mir-3648 (90nt per half hairpin). In such cases you may take more flanking residues. But these cases are rare.