I would begin by creating a survey, perhaps through SurveyMonkey, TypeForm or Google Forms.
Using the survey you want to create yourself a detailed user persona. So find out their painpoints, their desires, how they like to shop for their clothing, what style of clothing they're interested in, what do they want to know about tall menswear, would they like to get regular deals/info on tall menswear, find out what social network they spend time on, who influences them, etc, etc. Ask them at the end if they would like to subscribe to an email newsletter.
Of course you need to get responses. I would add an incentive for them to take the time to help you out. Obviously a tall mens clothing voucher makes perfect sense.
Promote it on your website/blog. Post the survey on your Facebook page and then boost it to your specific audience.
Then identify online influencers that might be interested in sharing your survey. If you have a budget advertise your survey on Adwords for relevant search terms.
The survey results will provide you with a detailed picture of your audience. From this you can create a range of blog posts that will directly answer their questions or solve their problems.
Then when you create each post, start promoting it!
Again get back to Facebook and promote it to your target audience. Advertise it on Twitter.
Check out QuuuPromote.
Repurpose the post and submit it to other sites as a guest post with your audience.