MR transfer behaviors of the permanent magnet biased spin valve MR sensors with SyAF (synthetic antiferromagnet) layers were studied by micromagnetics modeling. For narrow track MR heads, various height to width ratios were considered together with strength of permanent magnets which stabilities the free layed As the MR sensor width is reduced to $0.12 mu{m}$, sensor height less than 0.09 ${mu}{ extrm}{m}$ is needed to show good linearity and the Mr.t of permanent magnets smaller than 0.2 memu/$cm^2$ is sufficient for the domain stabilization. The conditions for single domain behavior of the free layer were also investigated through optimizing the biasing strength of permanent magneto the shield gap and the aspect ratio of MR sensor.