[spiel2001]

That would be [ Interface > AddOns > nUI > Layouts > Default > HUDLayouts > {hud mode} > nUI_HUDSkin_{hud mode}_{unit}.lua ] where {hud mode} is one of PlayerTarget, HealthPower, etc. depending on which HUD mode you use, and, {unit} is Player, Target, ToT, etc. depending on whose buff size you want to change.

Look for the ["Aura"] section in the file. Look for the following code block in each...

            [1] =    -- debuffs
            {
                anchor =
                {
                    anchor_pt   = "TOPRIGHT",
                    relative_to = "$parent_Health",
                    relative_pt = "TOPLEFT",
                    xOfs        = 40,
                    yOfs        = -100,
                },        
                options =
                {
                    enabled = true,
                    size    = 45,
                    strata  = nil,
                    level   = 1,
                    
                    aura_type        = "harm",
                    origin           = "TOPRIGHT",
                    player_auras     = false,
                    dispellable      = false,
                    horizontal       = false,
                    highlight_player = false,
                    aura_types       = true,
                    cooldown_anim    = false,
                    flash_expire     = true,
                    clickable        = true,
                    rows             = 3,
                    cols             = 4,
                    expire_time      = 10,
                    hGap             = 5,
                    vGap             = 20,

.
Changing the size value will change the size of the button... bigger values are bigger buttons. Keep in mind that making the buttons bigger means the whole box is going to get bigger and that means it will overlap the pet/vehicle name below it, etc. So you may have to play with it a bit. You can change the value of rows and cols as well which changes what the grid looks like.