Answer:
If a person is performing Salaat in a big Musjid, it will be permissible for one to walk two or three rows in front of the person who is performing Salaat. If it is a small Musjid, then it will not be permissible to walk in front of the person who is performing Salaat. A big Musjid is, at least, 235 square meters or more. (Ahsanul Fataawa vol.3 pg.409; HM Saeed)
Al-Mahmood 18