Are you placing a very small file (in terms of pixels), dramatically enlarging said file and then finding the pixels are blurring? If so, I believe your issue is related to Image Interpolation. When Photoshop scales content up or down in size, it can apply different algorithms that dictate how the new pixels appear.
The setting you are looking for is called Image Interpolation and it can be found in the General tab of the Photoshop Preferences.
When resizing, you generally want an option like Bicubic Automatic that will give smooth results. However, if your intention is to retain hard pixel edges, set Image Interpolation to Nearest Neighbor (preserve hard edges). Don't forget to revert this setting when you are done or any future resizing of photographs, for example, will give you crunchy results.
When you Place a file it comes in as a Smart Object. Everything that I have said above is true for the resizing of Smart Objects in general. If, however, you are Free Transforming a regular layer, note that an Interpolation option will be visible in the options bar running along the top of Photoshop.