Exploiting the optical/magnetic bifunctional upconversion nanoparticles (UCNPs) with small size has become a trend in the high-resolution biological imaging. Here, we report the simultaneous size manipulation and color tuning in Er3+-sensitized perovskite KZnF3 UCNPs via Mn2+ doping. It was found that the introduction of Mn2+ into the KZnF3 host matrix facilitates the reduction of the nanoparticle size (from 1 mu m to 50 nm). Under the excitation of both 980 and 808 nm continuous wave (CW) laser, these KZnF3:Er3+/Mn2+ (1/x mol%) UCNPs can efficiently generate tunable upconversion luminescence (UCL) and achieve pure red UCL as the Mn2+ concentration reaches 30 mol%. It is suggested that the energy transfer (ET) processes between Er3+ and Mn2+ ions contribute to the single-band red UC emission, which was well supported by investigating the dependence of UCL intensity on the pump power and the decay lifetimes of S-4(3/2) and F-4(9/2) states of Er3+ ions. In addition, the KZnF3:Er3+/Mn2+ UCNPs have been used as the T-1-weighted magnetic resonance imaging (MRI) agents, which exhibits a high optical contrast with the background. These features make the KZnF3:Er3+/Mn2+ UCNPs promising for excellent dual-modal agents in UCL imaging and MRI.