Hello,i am making alone my first 3D game. About 3D modelling i would like to make the textures but i have no clue about the size of textures. I know for one side using a small texture like 128x128, the game will work on lowend machines but with less realism, for other side if i use 512x512 textures it will have better realism but the game will not work on lowend. My target market is a indie game costing 10 euros. The models i will use are houses, trees and helichopeters. Any advice is welcome.

The minimum required supported size for a graphics card is 256x256, so that size or lower will work fine on any PC (Assuming Windows here).
According to the cardcaps spreadsheet, which comes with the DX9 SDK, there's not a single DX9 compatible card that doesn't support at least 1024x1024, and most support at least 2048x2048 or above.

Generally, you'd want to use the lowest texture size you can without losing quality; there's no point using a 1024x1024 texture for something that's usually 10x10 pixels on screen for instance.

Start with textures as big as you can conveniently make. Making the them smaller later is no problem, making the bigger, on the other hand, is.

256 is by far the safest size, and according to the Direct3D documentation, it's the fastest size too (if you have 4 128x128 textures, combining all of them into one 256x256 would actually be an improvement). EDIT: However, the documentation also states that textures must be kept as small as possible - meaning, 256² is fast, but only use it if you need that much size.

Also, make sure to work with squared textures; they're faster than rectangular (also taken from D3D documentation).

[Edited by - Kryzon on January 27, 2010 10:53:01 AM]

Thank you all for your advice