There aren't excercises to make an erection "harder". Some are just stiffer than others. The penis becomes "hard" when the spongy tissue fills with blood. No excercise will help that.
The posts you're reading, about girls being "too tight"? Those girls most likey weren't fully aroused when engaging in intercourse. The vagina is a muscle, which expands and contracts. It has the ability to pass through a baby, and I assure you there are no penis's larger than that. So, when fully aroused, "tightness" isn't an issue, because the vaginal walls will accomodate any size penis, as far as width. Lenghtwise, the vagina is not a never-ending tunnel. Most average 5-6 inches and all end with a cervix.
To make intercourse and all activities pleasurable for you and her, ensure there's plenty of foreplay and make sure you are aroused. Take a look at our article, Orgasm and Sexual Response.