When u press "render" button, Maya will render your current active panel. May be u have active other Orthographic panels at that moment.
OK, the other question is u click the "Render View", right?
In "Render View", u can "click & drag" to select a render region. If u just click it once, Maya assume u have selected a tiny bit render region. May be the area just have 5 x 5 pixels, so u get the warning - "render view:selected region is too small ".
P.S. I'm still in office, so English typing is better than Chinese.
