There is an issue raised by the poskim that having long hair is a chatizah with regards to tefillin.
See the following secondary sources:
Notes on the Mishna Berura by R'Yosef Rimon:
The Weekly Halacha by R'Daniel Neustadt:
Lettings ones hair grow long a shiur by R'Aharon Lichtenstein:
Shiur by R'Eytan Weisberg
A couple of questions:
1) What about the issue of the nazir, surely he had long hair and yet had to put on tefillin? Asides from the nazir, was it not common in biblical times to have long hair and in more contemporary times rabbis like the Rogachover Gaon and the Chatam Sofer had long hair?
2) In halacha everything has a "shiur", has anyone seen any measurements of what is considered long hair?
My understanding is that the issue being spoken of is a comb over, where the hair from the back of the head (i.e. hair that is growing from a position that you cannot place the tefillin shel rosh is pulled to the front). I would assume it is not an issue if the hair is long in the front, provided that the hair is from a position where you can place tefillin. Eg this picture: