1:1 icon on the map toolbar, or "Modify, Map Area, 1:1 view" menu command, or from "Zoom" on the pop up menu when you right click on a map. .
You can get true 1:1 maps, where every pixel (data point) on the map displays on screen, only for maps where both the x and y spacing are identical (most older USGS data in UTM projection) and for maps where the X and Y spacing varies (almost all NGA data and now the USGS seamless data, with constant spacing in arc seconds, which varies by latitude when converted to meters). Arc Second data will have some pixels displayed twice so that every pixel displays and scaling remains correct.
You can get a custom map size from "Zoom" on the pop up menu when you right click on a map. This will be the linear zoom factor compared to 1:1. It will be exact for a UTM grid, but only approximate for lat/long data.
Last revised 8/23/2016