A deuteron NMR study of a pentylcyanobiphenil (5CB) liquid crystal confined in controlled pore glass (CPG) matrices of different pore diameters 2R between 7 nm and 400 nm has been performed. The paranematic-nematic (P-N) phase transition seems to be gradual in pores with 2R less than or equal to 24 nm and discontinuous in larger pores. Possible reasons for the observed temperature shift Delta T and qualitative change of the phase transition are analyzed using a simple phenomenological approach. The theoretical analysis suggests that both surface interaction and elastic distortions significantly influence the Delta T = Delta T(R) dependence obtained in CPG samples.