As a rule of thumb - for most rifles and most 30mm optics, which will be 40,44, or 50mm objectives, medium rings will work fine.
What diameter of objective did you buy? The 30mm is the tube diameter, so you'll need 30mm rings, but you'll need to match your objective diameter to your ring height.
What bases are you using on the rifle?
A couple tricks for determining ring height:
1) Quick and easy measurement: Cut a deck of playing cards into 2 sets of 1" wide strips. Set two stacks of EQUAL NUMBER of cards on top of your bases in position of your rings (start around 1/2" thick). Add or remove cards - from both piles equally - to raise or lower the scope to give your desired objective bell and bolt handle clearance. In general, I want 2mm to 1/8" clearance up front (which can also be measured with playing card "shims"). By starting at 1/2" thick, for a sporting rifle, you should only need to remove shim cards, not add. Once the clearance is correct, measure the thickness of the card stack, and order rings of approximately that height. So say I find I need 0.40" of card shims to get the right height, I know I need ~0.40" rings, from top of base to bottom of tube.
Take care here - some rings are sold with their heights measured from the top of the base to the CENTER of the tube. For these, you need to subtract half of the diameter of the tube, and compare THAT to your card stack height.
2) More work/math: Hold your scope up beside your rifle as if it were mounted, so you can then tell where the objective bell will hang over the barrel. Lay the rifle upside down on the bench so it's flat on its base(s), then measure the gap under the barrel where the objective bell will hang. (This can be measured by sliding stacked feeler gauges or playing cards, and measuring the thickness). Say the base height is 1/4" over the interference point - so 0.250". You should be able to look up or measure the diameter of your objective bell, for example, my Leupold VX3i 4.5-14x50mm objectives are 2.3" dia. The 30mm main tube is 1.18", so the objective is 1.12" bigger than the tube, so the tube needs to be lifted 0.56" (half of 1.12") to have the bell just touching a surface. In general, I want 2mm (0.0785") to 1/8" (0.125") clearance under the bell, so my scope needs 0.56+0.079 = 0.639" to clear a flat surface - but remember, our action, barrel, and bases aren't a flat surface. Since I have 0.250" depression from base to barrel, I can subtract: 0.639-0.250 = 0.389". Nightforce standard 30mm rings are 0.9, 1, and 1.25" for low, medium, and high rings, from base to center, which is 0.31", 0.41", and 0.66" from the top of the base to the bottom of the tube... So... Low rings aren't tall enough, but Mediums are perfect to give me between 2mm and 1/8" clearance.
NOW... Neither of the above give me any idea for bolt clearance relative to my eye box diameter, so that's something I have to watch when I try installing the rings.