All your questions would be better asked IDM support by email than asked in user-to-user forum as the developers of IDM Computer Solutions, Inc. definitely know the icon layout guidelines better than any user.
However, I looked with Resource Hacker tool on resources stored in ueres.dll and extracted some of them for further investigation as already done in the past for example for Previous icon set available in UltraEdit v17.00?
and Icon size of toolbar symbols beyond 24x24 pixels for small screen with a high resolution
Resources of UltraEdit for Windows < v23.00 and UEStudio < v16.00 contained the icons for the toolbars as bitmaps (*.bmp) in the dimensions 16x16 and 24x24 pixels. The color of top left pixel was used as transparent color. This was also used for user and project tool icons in the toolbar of UE < v20.00.0.1052 or UES < v14.00.0.1012 even if the image format was GIF, ICO, or PNG with having a color defined as transparent color. BMP and JPEG are not supporting transparency at all. For more details about rendering of user/project tool icons in older versions of UE/UES see Colourless Icons
Since version 23.00 of UltraEdit for Windows and v16.00 of UEStudio the command icons in the resource DLL are stored in format PNG
in true color with using an alpha channel
(32-bit color depth, i.e. 24-bit for RGB and 8-bit for transparency).
A portable network graphics image with an alpha channel is of course very good for icons on ribbons, menus and toolbars.
The icons for the toolbars are drawn in the dimensions 16x16 (small toolbar) and 32x32 (large toolbar). For the ribbon tabs there are also 64x64 icons in the resource DLL which are adapted in size according to screen resolution.
So the best image dimensions for a user/project tool depends on where the user/project tool icon is displayed: on ribbon with a large or small icon, in a menu or on small/large toolbar.
The ICO file format
supports multiple images in different dimensions stored in same *.ico file without or with a transparent color and without or with an alpha channel.
Some general graphic symbols on ribbon are stored as *.ico in the resource DLL each containing 1, 2 or even more images mainly in the dimensions 16x16, 32x32 and 64x64.
But I have not tested if UE v23.00+ or UES v16.00+ take the most suitable image within an *.ico file on loading a user/project tool image for ribbon/menu/toolbar as it is done by Windows for example for the icon in title bar of a window.
I have used always *.ico files with having a transparent color defined for the symbols of my user/project tools in the last 15 years. I have not tested if ICO images with an alpha channel are also supported by UltraEdit/UEStudio now or perhaps already in older versions of UE/UES.
So my suggestion is using either ICO or PNG with at least a transparent color defined, or even better with an alpha channel if the symbol has curves and uses anti-aliasing or shadings, in the dimensions as displayed in UltraEdit/UEStudio.