Large body of empirical literature has been published, investigating determinants of entry (Audretsch and Acs, 1994; Mata, 1993) and life duration of new entrants (Audretsch and Mahmood, 1995; Weiss, 1998). A topic that has received much less attention is the start-up size of firms although it has been proved as a significant determinant of survival and growth. A few exceptions are studies by Mata and Machado (1996), Gorg et al (2002), Resende (2007), Capelleras (forthcoming). But none of them took into account the transitional and marginalized context. Departing from the limited supply of the empirical evidence deriving from transition countries, the purpose of this paper is to identify, and compare the determinants of start up size and post entry growth. In addition we analyze the behavior change of firms, with a special focus upon the main job contributors – fast growing firms.