Definitely you need enough butter for the flour, or of course, the whole thing will just be crumbs in the pan and won't bubble or cook, but just burn as clumps of slightly greasy flour - If you do equal parts by volume of flour and butter, you should be ok, it will be a little more liquid in that case and easier to make. If you want to follow the recipe precisely and you don't have a scale, you can go online and write in the search bar: conversions butter flour weight volume on google and find plenty of sites that will give you the precise equivalent of weight to volume (tablespoon, cup, etc). But you can also put the butter in, melt it over low heat, and add the flour a little at a time, mixing, till it's still homogenous and doesn't become a 3 dimensional clump.
Another trick in using roux is to heat the milk or other liquid (broth) separately, take the pot with the roux off the fire, and plunge the bottom of the pot into a dish of water to stop the cooking. Add the hot milk all at once and whisk and then proceed to cook it. Not necessary, but useful for a beginner, and prevents lumps in the sauce.
The proportions don;t have to be exact, but some will give you a thin sauce, some a thicker sauce, and it depends on what you want to use it for. To make croquettes, you want a very thick sauce, to make a sauce to pour over something, you'll want it thinner, to make a sauce to bind a vegetable dish with eggs and breadcrumbs and grated cheese to bake in the oven and then slice, you'll need a medium thickness.