photo paper usually comes in A4 measurements. But there are some that comes in letter sizes too. letter sizes (8.5 x 11) is US standard while A4 (20.99cm x 29.77cm) is I think UK standard. 150 gsm I think would be the thickness of the paper.
the brand quality will depend on how meticulous you are. Eyes for colors and etc. And usually those who are in print keeps a close look on those colors. if you are not really particular with color the generic ones will do, however make sure to check your settings on your photo editing softwares.