I've printed up to 20x30 on my D300 that turned out great! But the D5300 might be able to produce better photos, though the D300s is a pro model and much better built with the pro style button controls layout! For build quality the D300s is a much better camera, but for new tech advantages the the D5300 is better from what I hear!
But I would recommend a D7100 instead of the D5300 if higher MPs is your biggest need, so you still get a simi-pro style camera with the higher MPs!
The D7100 is cheaper now that the D7200 is out!
The D300s would be my favorite DX choice, due to excellent design,feel,and built quality!
This the photo I printed 20" x30"!