By "page", I take it you mean browser window.
Once open, click-hold-drag the top part of the window to position it where, on the screen, you want it placed. Then, click-hold-drag the bottom right corner where you see small diagonal lines to resize the window to what you want.
If the window is too large to grab the bottom R corner after *positioning* the window, then resize first.
Quit the browser. When reopened, the window should be as and where you last had it.
Doing all the above may not fix the problem of some page/window displays requiring sliding the horizontal adjustment across to read everything. It's irritating, I know, but it depends on how the page is designed. There may be something in your browser preferences (proportional font?) to adjust page display. With Camino, my preferred browser, resizing this M-F page for example also adjusts the way the page displays, and I don't have to use the horizontal slider.