First, electrons should go perpendicular to both electric and magnetic fields in order to not be deflected. Also, the field and velocity directions are important. In Cartesian coordinate system, if electron should travel along x axis (for example), then direction of eletric field (from '+' to '-') lays along y axis and direction of magnetic field lays along z axis.
Electrons would then pass undeflected if electric force F1=eE is equal to magnetic (Lorentz) force F2=evB. F1=F2 yields to eE=evB, so finally we get v=E/B where E and B are magnitudes of electric and magnetic fields, respectively. v=3,5*10^6 m/s.
If electric field is turned off and velocity of elctron beam is the same then centripetal (deflecting) force F3=mv^2/R would be magnetic force F2=evB, F2=F3. So mv^2/R=evB gives R=mv/(eB) where m - electron mass and e - electron charge. This is approximately 5,7*10^-3 m if I'm not mistaken.