You can open the tileset in an image editing software like the free GIMP and then choose "Image > Scale Image..." and scale the image to the size you want with "Interpolation" set to "None", so it doesn't get blurred. Please make sure to resize to a multiple of the original size (32px, 64px, 128px, ...). If you want to resize not the tileset but a lot of the single images, there is also a plugin for GIMP called BIMP to batch-resize a lot of images at once. If you want, I can guide you through that, too. Valuable editing skills for game development! ;D
You can also send me a mail at email@example.com and I'm sending you the scaled images back (do you need only the big tilesets or the single images or both? Which size?).