Sorry for the late reply. I haven't been sleeping well and have been completely useless the past couple of days.
Anyway, to answer your question about blurry textures, it's not the size of your texture you should worry about. It's the size of your UV islands. Bigger UV islands = better textures.
Your UV layout isn't right. I had a devil of a time but I managed to export the game's Ladderlike Stair's meshes and the Townhouse Stairway meshes in blend form (the two stairs
this tutorial says to clone).
They are here. Open each of the blend files in the game. Note that the center portion of each mesh has a UV island that extends from one edge of the UV map to the other. Yours has to be the same. Study how EA has made their UV layout and try your best to mimic it. Since you will have to change the UV layout this means that you will have to change your diffuse, as well.
While you're looking at the EA meshes' blend files, put the meshes in vertex paint mode and take a look at how the vertex painting is done. Only the blend files will show the vertex paint; WSO files won't preserve vertex paint when importing to Blender so the vertex paint is messed up with those files. This is why I extracted the meshes as blend files.
Since your UV layout has to be done a certain way, it helps if you assign your mesh to a colored-squares background like
what's in this tutorial. Select your mesh, edit mode, in the UV editor press A to select all of the UV islands. Near the bottom, press the button that says "New." A little window will pop up. Set the width at 1024 and the height at 2048. You're making this texture this size because your diffuse is a rectangle so the colored square texture should be a rectangle, as well. Otherwise, your UV islands will be misshapen once you import the mesh and texture to your file. In the dropdown menu, choose Color Grid and press OK.
Your mesh will be assigned to this texture now. To see it on your mesh, enter texture viewport shading. Press N to open the side panel. Under Shading switch the dropdown menu to Multitexture. Now, if you edit or move your UV islands you will see how the texture looks on your mesh in real time and you can see if the texture is stretched or uneven, etc., and you can easily fix any problems. Press N to close the panel again.
Note if your mesh has been vertex painted the vertex paint will show once you've changed the shading to Multitexture. This is so annoying. I don't know how to fix this problem yet, sorry. You can remove the vertex paint but obviously removing vertex paint means you'll have to redo it later on. Looks like your mesh has been painted black so it's no big deal to redo that later if you need to. In the Data tab, where it says Vertex Colors, select the vertex color entry and press the minus sign to delete it.
One other thing to mention, it looks like your stairs have been vertex painted black but the landing has not. It still has the EA vertex paint. You will want to fix that so that it matches the other meshes.
The last issue is that your mesh's polycount is too high. It's not horribly high—I've seen much, much worse—but there's really no reason for it to be as high as it is. Higher poly doesn't always mean a better mesh; you should always keep your polycount low. Helps prevent lag. The cleanest way to reduce polycount is to dissolve edges; the decimate modifier will kill your topology so I don't use it if I don't absolutely have to. Select your mesh, enter edge select, hold shift + Alt and select some edge loops, X > dissolve edges. Focus on the areas that are dense. You can remove a lot of edges and your mesh will not change its shape.
You WILL have trouble selecting edge loops. The reason is that there are so many unmerged vertices. You can enter edit mode, press A, W > Remove Doubles but then you'll need to
split some edges later to avoid any ugly shadows. Since you have to redo the UV maps anyway I say it's worth it.