May one eat while wearing Tefillin?
A meal: It is forbidden to eat a meal with one’s Tefillin due to worry that one may come to get drunk and belittle [himself] while wearing his Tefillin.
A snack: One may eat a snack while wearing his Tefillin.
What is defined as a snack and what is defined as a meal?
The definition of a snack and a meal are the same as defined regarding the obligation to eat in a Sukkah. A meal is defined as eating a Kebeitza [55 grams] of Mezonos or Hamotzi, and thus may not be eaten with one’s Tefillin. A snack is defined as eating any amount of Borei Nefashos foods, or less than 55 grams of Mezonos or Hamotzi. These foods may be eaten with ones Tefillin.
 Admur 40/11; Michaber 40/8
 Admur ibid; Michaber ibid
Other opinions: The Mishneh Berurah [140/18] brings opinions which question whether this allowance to eat a snack applies even today when we no longer wear our Tefillin throughout the day. In Piskeiy Teshuvas [40/8] he rules [based on the M”B] that if one plans to remove his Tefillin already, then he is not to eat or drink at all while wearing them. If however he plans to wear them and thus desires to eat and drink now in order to have strength to learn or pray, then he may do so, and so was the custom of Gedolei Yisrael [Chasam Sofer; Mahrahm Shick].
 So rules M”B 40/20, and so seems to be the opinion of Admur which uses the terms of Keva and Aray whicha re also used by Sukkah. However there is an opinion [brought in Piskeiy Teshiuvos 40/8] which sides that one may eat Mezonos just up to the point that it is considered a Kevius Seudah, which is up to 4 Kebeiytzim.