For .dds files, resolution doesn't matter. It's pure pixels... if you're starting from scratch and your graphics program demands a resolution, just use 72, which is standard screen resolution.
Have you unpacked your BSA? All existing graphics (and other resources) for Oblivion are packed into BSA files in the [Oblivion\Data] folder. You'll need to unpack (using an outside program called
BSA Commander) the file called [Oblivion - Textures - Compressed.bsa]. From there you can start browsing and opening the texture files yourself.
I opened the acrobatics class images to let you know the sizes. You can use them as an example of how to size them... but it would be best to just use the originals as templates.
[All sizes listed will start from the upper left corner of the same file.]
Oblivion\data\textures\menus\class\attributes\load_image_acrobatics.dds
image size:
w=512
h=512
background paper texture:
w=410
h=410
actual icon size (and white area in the Alpha channel - has some black details):
w=240
h=344
Oblivion\data\textures\menus\class\attributes\load_image_acrobatics_small.dds
image size:
w=64
h=64
icon without the border outline (not required, except for consistency with the other icons):
w=56
h=56
Alpha channel (all white):
w=64
h=64
There is also a resized copy of each of these files in the folders menu80 and menu50. As the names suggest, they are scaled down (for different screen resolution settings that Oblivion runs in) at 80% and at 50% of maximum.
Oblivion\data\textures\menus\class80\attributes\load_image_acrobatics.dds
image=512x512
background=328x328
Alpha channel/icon=192x276
Oblivion\data\textures\menus\class50\attributes\load_image_acrobatics.dds
image=256x256
background=205x205
icon(no border)=120x172
Oblivion\data\textures\menus\class80\attributes\load_image_acrobatics_small.dds
image=64x64
Alpha channel/icon=51x51
Oblivion\data\textures\menus\class50\attributes\load_image_acrobatics_small.dds
image=32x32
icon(no border)=28x28
Like I said, it's best to just edit your new image on top of the old file, then flatten it and save-as your new texture... unless you're just trying to make new versions of the old icons... in that case just save right over them. I would advise extracting these BSA textures to a separate folder for editing and only moving the finished, working textures into your Oblivion data folder for in-game testing. As long as you make your file structure in the Oblivion data folder the same as it was created in the BSA, any files in there will be used instead of the typical Oblivion textures. There is no need to extract all the files to your Oblivion data folders, only the files you're attempting to replace.