There are three things to look at when choosing paddles - paddle length, blade type and grip type. Shaft+grip length is based on you+your boat, blade type on the water+trip type.
Shaft+grip length is the key to overall paddle length. Blade lengths vary widely based on blade type, but shaft+grip is consistent for a given paddler and boat. It's the length needed to fully immerse the blade when the paddler's top hand is about chin level.
The best way to get shaft+grip length is to get in your boat on the water in your normal paddling position and measure it. If that isn't possible, there are ways to estimate it - see the article below. My paddles range from 56" to 61" overall length based on blade.
Once you've established your shaft+grip length, overall paddle length is easy - it's your shaft+grip plus however long the blade you want is. Stern paddlers may want to add an inch or two for steering. Bent shaft paddles are mostly an inch or two shorter.
A lot of variety here to support different paddlers, paddles, and water. Blades for longer trips and/or deeper water tend to be longer and narrower. Blades for faster and/or shallow water tend to be shorter and broader. Again, see article below.
Grips come in two broad categories - "palm" and "T"
"Palm" grips are the most common. There are sub-types but all share being rounded to fit in your palm and flowing smoothly into the shaft. This type of grip provides a lot of flexibility in handling the paddle, encouraging use of a variety of strokes.
"T" grips look like they sound - a cross piece perpendicular to the shaft. Mostly used in specialized whitewater and children's paddles. Give the firmest grip, but less flexible.
Here's a good article: