Premiere Create custom frame size
What you want to do is make a sequence with the same frame size as your source file (in Premiere and other NLEs your project doesnt have a specific frame size, as you can have multiple sequences with different frame sizes in one project)


Open Premiere and Start a new Project (File > New > Project)
Give the project a name and click ok (dont worry about any of the options)
In the New Sequence window click Cancel - this will take you straight into the project without making a sequence
Import your footage (file > Import)
Create a new Sequence from clip by either:


Dragging the footage down to the New Sequence icon (in the project panel) OR 

Right click on the footage and select New Sequence from Clip




This will create a sequence that matches the attributes of your footage (frame size, frame rate etc)