Having a foreskin will not restrict the length of your penis in any way, nor its basic girth. However, even a moderately tight foreskin will inhibit the expansion of the rim of the glans to its full potential.
Your foreskin should have become fully, freely and painlessly retractable (whether flaccid or erect) by 8 years old and your parents ought to have ensured that you could freely retract and taught you to do so every day to wash well under there like the rest of your skin.
As a teen or adult it is absolutely vital that you can, and do, fully retract your foreskin at least daily and wash away, with soap and water, all the the smegma and stale urine (both totally waste materials) that constantly collect under it. Failing to do so leads to objectionable odours and eventually to serious infections. Some, like penile cancer, are so dangerous that they can result in the partial (or even complete) amputation of the penis (which really does seriously shorten its length!)