File>New Document
In the Page Size area specify this, you can use inch marks to indicate inches, indesign will use inches, just convert them to the unit you have selected.
If you want to use inches for all new documents, before you open or make a new document. You need to open InDesign, without any files or documents open. Then in the Edit>Preferences>Unit & Increments>
Select the Unit of measurement you want to use. This will apply for all your new documents. Any documents you have saved previously will use the size that was used then.
FYI
1 pica = 12 pts.
There are 72 pts in an inch
1" is equal to 25.4mm
A A4 is 210 x 297mm (european)
So 49p6 x 75p11 equals 210 x 297 mm
49p = picas and the 6 represents pts. 49p6
75p = picas and there are 11pts. 75p11