Creating large scale printed posters

Hi, I'm a relatively new artist to Fresco and I've really come to love it.  I've used it to create some great pieces, but it seems to have limitations for print.  So far, I have started each drawing at 18x24 300 dpi (the largest option) on Fresco.  Once I'm done painting the piece, I open it in photoshop and change the image size to a larger size (with automatic resample).  I've had success printing those images up to 27x36.  Some are clear, but many are pixelated and have to be printed smaller (although still larger than the original 18x24).  I prefer to use the pixel brushes, although I know that vector brushes are better for upsizing.  

My question is, am I approaching this right?  Is this a logical way to be starting a painting intended to print at a larger scale?  I really would like to continue working in Fresco if possible (photoshop is a little overwhelming).  I've found that some brushes upsize better than others, but it's a bit of trial and error.  Anyone have any brushes that they've found print well at a larger scale?  

Any input would be great!  I'm new, not terribly tech savy and am hoping to be able to create and print my art as smoothly and simply as possible.