Getting the right size planters is really important as it can heavily impact the health of your plants. You need a pot that is large enough to comfortably accommodate your plants, otherwise you risk stunting your plant's growth, but it also can't be too big as that increases the chances of root rot and bad soil, due to the soil not drying fast enough.
When picking a planter, measure the diameter of your plant's roots and the planter itself. You'll need to pick a planter that has a diameter that's at least 1.5 times bigger than your plant's root diameter.
So, for example, if the diameter of your plant's roots is about 4 inches, you'll need a planter with a diameter of at least 6 inches.
Your plant will almost definitely outgrow its current planter, so you need to be prepared to re-pot it at some point.
If it's a slow growing plant, you'll only need to size up by 1 inch or 2 inches in diameter, but if your plant grows at a pretty fast rate, you'll need to size up by 2 inches to 4 inches each time.
For more stationary planters, like raised beds, you'll need to do a bit of planning ahead - work out the rough diameter of each plant, then leave at least that amount of space between each plant, more if they're fast growers.
For example, if your plants have diameters of 2 inches, leave at least 2 inches between each plant within the raised bed. This allows them to safely grow without invading each other's root space.
If you're growing plants with shallow roots, like vegetables, you can get away with having a pretty short raised bed of only about 10 inches in height.
If you're worried about rodents getting to your plants, invest in a raised bed that's at least 18 inches in height - things like mice or rabbits won't be able to easily reach your plants and will soon give up trying.
For older individuals, or those suffering from back problems, it's recommended to get raised beds that are at least 22 inches in height. This higher elevation will prevent you from having to bend down really far when gardening, reducing the risk of back pain.