I'm currently coding a mini game (no big deal, just for fun )
And I would like to use some sprite, but I think i dont undersatnd everything ^^.
How could I know what it's the size of the square of each picture, and where it begin and end ? The probleme is that for an animation it's not the same for each picture.
For a animation of 10/20 picture I could not find each hand by hand ! I think it must have a size and an offset who is the same for each picture of one animation.
Animations like this are hard to arrange in a way to make it look good; though it can be made with some techniques. Good animation programs have a onion skin feature (which makes the frames transparent, helping you figuring out where the next frame should be positioned). If it can show animation previews, it'll help you, though not as much as the onion skin. If you don't have an animation program that does that for you, you can always deduce it.
First of all, find a maximum size of your animation. Check the animation's frames and see which frame has got the bigger height and which frame has got the bigger width. After finding those values, compare both numbers and see which is bigger (Example: if the maximum width is 20 and the maximum height is 13, choose the width). Use the bigger value to draw a square. This will be the animation's boundaries. Sometimes it won't work, but this is a rough estimative. Increase its value when needed.
After creating the box, start placing the sprites with a fixed point in mind. For example, when making an idling animation, try placing the character's feet in the same place.
Though it's not THAT accurate compared to the onion skin/preview method, it helps you considerably.
i'd recomend using Animation Shop 3, it comes witha built in onionskin feature and its quite simple to work with.
and i realy recomend you to know what you're doing. this is, knowing how the sprite is animated so you know where and how each sprite is placed in your frame. use a bigger one so you have more room to move the image around until you're done and just crop all the unnecesary canvas.
I don't know to answer the first question, but if you meant 'why don't everybody put the rips in uniform boxes so animators could use them without trouble' in the second question, it's because ripping/customing is often a time-consuming activity. Calculating the canvas size for every animation would just make the process longer.