Free electron lasers (FEL) need a very bright electron beam in three dimensions and a high peak charge density. In order to compress an initially longer electron bunch generated from the photoinjector, magnetic bunch compression systems are widely employed. In this paper, first harmonic RF linearization and its associated requirements are reviewed. Meanwhile it is also briefly discussed what is the relation between a proper initial bunch length and main RF frequency, when a harmonic RF linearization is included. Then given a reasonable bunch compression ratio, a proper initial bunch length as a function of the main RF frequency and RF phase is estimated analytically by several approaches, assuming that no harmonic RF section is needed to linearize the energy modulation introduced during main RF acceleration, and at the same time still linearly compress the bunch length. Next the upper limit of the bunch compression ratio in a single stage is evaluated analytically. The analytical relations derived on choosing a proper initial bunch length as a function of main RF frequency are confirmed by numerical simulation. These simple limit provide rough estimations and may be beneficial for choosing bunch compression ratios in different stages of an FEL driver, especially in a first stage bunch compression where there is usually a harmonic RF linearization applied. It may also be useful in evaluating the possibility of low charge operation mode without any harmonic RF linearization, where a shorter initial bunch length can be achieved from the photoinjector. Show Partial Abstract