Blender has tools to do it without decimating as in case of more complex models this changes geometry and should be used only if it is intended. Alternative way is to zoom in with e.g. the mouse wheel prior to selection.

Blender has tools to do it without decimating as in case of more complex models this changes geometry and should be used only if it is intended. Alternative way is to zoom in with e.g. the mouse wheel prior to selection. Or, in Edit mode, press A to select all.