Computers are notoriously bad at computing with decimal numbers. That includes most programming languages.

If you want accurate arithmetic, you might want to try using Perl 6:

> my $size = 0.01;
0.01
> my $step_size = 0.01;
0.01
> $size += $step_size while $size < 5;
Nil
> say "FINAL: $size";
FINAL: 5
[download]

;-)