I've been to a lot of scientific conferences and see 10-20 passengers all getting on with their posters in cardboard tubes (along with their other carry ons). Never seems to be an issue really, ESPECIALLY if you plan on putting your larger carry on and purse under the seat in front of you.

Worst case... just stick the poster as far into your purse as you can and leave it hanging out about 30". Now it's just one item