My recorded video is 1920x1080, but Premiere Pro won't let me export anything larger than 352x288

Hi all!

I'm afraid I'm a bit of a novice when it comes to video editing.  I recorded some video using a Nikon SLR camera.  According to Premiere the properties of the imported video file are:

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Type: MPEG Movie
File Size: 119.44 MB
Image Size: 1920 x 1080
Frame Rate: 29.97
Source Audio Format: 48000 Hz - 16-bit - Stereo
Project Audio Format: 48000 Hz - 32 bit floating point - Stereo
Total Duration: 00:01:26:09
Pixel Aspect Ratio: 1.0
Alpha: None
Video Codec Type: MP4/MOV H.264 4:2:0 (Full Range)
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But when I try to export the video as an MPEG4, which is the required file format, it won't let me export with anything larger than 352 x 288 aspect ratio.  I've tried changing the sequence settings but usually get a pop-up saying that the video doesn't fit the settings.  If I keep the settings anyway then the video exceeds the Preview display size.  The sequence settings I'm running with currently are:
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Editing Mode: Custom

Timebase: 29.97 frames/second

Frame Size: 1920 x 1080

Pixel Aspect Ratio: Square Pixels (1.0)

Fields: No Fields

Preview File Format: I-Frame Only MPEG

Width: 1920

Height: 1080

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What am I doing wrong that makes it so that I can't export an MPEG4 at a higher aspect ratio?  Any help would be greatly appreciated!  Thank you!