Why does Photoshop image pixelate when I place it in a 145 by 145 image size?

To add to ​@Conrad_C ‘s excellent reply, check your image at 100% zoom (Ctrl+1). In Photoshop that is not a physical size but simply means 1 image pixel mapped to 1 screen pixel. The physical size will depend on the screen in use. At that zoom level it should not look pixelated, although the image quality may be degraded, in comparison to a larger image with more pixels, simply due to the use of just a few pixels to display it.

145 x 145 is not a lot of pixels. It’s equal to 0.02 megapixels. 

Whether it looks pixelated depends on how large you are showing it. For example, a common desktop icon might be only 256 x 256 pixels, but it looks sharp because on a Retina/HiDPI display it might be over 200 ppi.

This is not even a digital issue, because if you were given square stone tiles and had to arrange them into a 145 x 145 mosaic, that would look pixelated too, even though it’s not on a computer.

So, at what magnification are you viewing it in Photoshop, and is that the final scale of the 145 x 145 px image? Because if it’s going to be shown at 1:1 on a Retina/HiDPI display then it should look sharp enough, but if you are looking at it in Photoshop at 800% then of course it’s going to look pixelated because you zoomed in. 

What size are you starting from? How complex is your image and what interpolation method are you using to downsize? Its normal for images to degrade when sized way up or way down, if its a big image and you shrink it, there are simply fewer pixels left and its much harder to keep it looking sharp.