To create textures, you need to work with the power of two. This is due to how computers work out the different equations in the engine which means that if you did not use the power of two, the game would work, but be less optimized than if you would have used it. Of course this is extremely important if you want what you are making to run well and when you consider all the textures within a game it makes a big difference in the end. You first decide by what resolution you want to use that is within the power of two. The higher the resolution the more load it is on the engine, so you have to think about the hardware you are using and whether or not in can handle it. For example, if your game was to run on pc you could potentially put higher textures into the game due to the extra processing power than say a mobile game. You also have to consider what is worth texturing which means that you have to think whether or not the player will come into close proximity of it and get up close to it. If they could consider giving it a higher resolution. But for some things it is not worth it. For example, a fruit bowl with individual bits of fruit, lets say grapes; giving them a higher texture is not worth it because the player is less likely to encounter it and look up close which means that the computers resources is potentially wasted and could have been used elsewhere.
Once you have put your image into Photoshop you must then decide what resolution you want to use within the power of two. Once you have a set resolution, create a new document and half the resolution and copy a fixed size of the image (half resolution again) then put it into your new document. Clean up the image if necessary. Then go onto offset and half the resolution again for vertical and horizontal which will allow you to to blend the texture together using clone stamp. Once you are happy with it and decide everything looks cohesive and seamless, create a new document and depending of the resolution you are working in, double it then copy and paste the textures into the new document. Now you have your final texture.