Zoom to Fit - This will shrinks the output TexToPix image to display panel.
Actual Size - This will display the TexToPix image in actual size.
Zoom In / Zoom Out - Use these to control the image zoom.
Please remember that the preview shows a bitmap based version of the image. It's resolution is limited with bitmap size that in it's turn is defined by Base Font Size and Number of characters in line.
When you export the TexToPix picture into PDF file you are not limited with bitmap resolution because your text characters will be drawn as a vector shapes that could be losslessly scaled to any need size.