Abstract All-optical image processing based on metasurfaces is a swiftly advancing field of technology, due to its high speed, large integrability and inherently low energy requirements. So far, the proposed devices have been focusing on canonical operations, such as differentiations to perform edge detection across all objects in a complex scene. Yet, undesired background noise and clutter can hinder such operations, requiring target selection with digital post-processing which inherently limits the overall accuracy, efficiency and speed. Here, we introduce an optical solution for real-time size-selective image processing and experimentally demonstrate the concept with a metal-dielectric-metal film performing a spatial band-pass filter in momentum space. We show high-resolution (~0.9 μm) edge detection and real-time dynamic denoising, ideally suited for bio-imaging applications and target recognitions. Our demonstrated k-space filtering metasurface expands the scope of nonlocal flat optics for analog image processing, ushering in opportunities for ultra-compact, cost-effective, and multifunctional image processors.