If you want to use the tileset in fullscreen mode, locate and modify the [FULLSCREENX:800], [FULLSCREENY:600], and [FULLFONT:curses_800x600.bmp] configuration lines to match the specifications from the tileset list. The FULLFONT directive should match the filename of the tileset you downloaded. If you want to use the tileset in windowed mode, search for the [WINDOWEDX:640], [WINDOWEDY:300], and [FONT:curses_640x300.bmp] lines instead, and change them to the correct values.
The 800 and 600 values are dependent on the size of your new graphics and the size of your screen. A tileset with 16x16 tiles will be 256x256 pixels large, and the target resolution will be 1280x400, unless you change the tile width and tile height in the same file.