I suppose, you mean the two arrow buttons on splitter between the text comparison panes in UltraCompare Professional v18.10 for Windows.
It is not possible to change the size of those buttons/symbols/commands respectively the width of the splitter as far as I know. It is only possible to hide them by unchecking setting Merge buttons on splitter at Display - Text Miscellaneous in configuration opened with a click in menu Options on menu item Configuration on using Toolbar/Menu Mode or with a click on ribbon tab Home in last group Configure on item Settings on using Ribbon Mode.
There are on ribbon tab Edit in fourth group Merge the items First to second and Second to first on using Ribbon Mode.
There are on toolbar Merge the buttons Merge First to Second and Merge Second to First on using Toolbar/Menu Mode. The size of the buttons on the toolbars in Toolbar/Menu Mode can be modified by clicking with secondary (right) button on any toolbar to open the context menu and click on context menu item Large Buttons.
Switching between Ribbon Mode and Toolbar/Menu Mode can be done by clicking with secondary (right) button on ribbon/menu/toolbar to open the context menu and click on context menu item Toolbar/Menu Mode or Ribbon Mode depending on which user interface mode is active at the moment.
The commands can be also executed by keys. Merge First to Second is by default assigned to Alt+Right Arrow and Merge Second to First to Alt+Left Arrow. The hotkey assignments can be changed in configuration at Key Mapping. The command Next Difference has the hotkey F3 and the command Previous Difference the key F4. I like using F3/F4 and Alt+Left/Right Arrow and finally Ctrl+S for merging text differences.
Best regards from an UC/UE/UES for Windows user from Austria
Thanks Mofi for your very informative reply. I guess this should be a wish then. I'm often comparing modified source code against newer versions of the source code to see what's been changed/added, so I click on the Go to Next Difference arrow, click on the block of code I want to copy from new source code, then click the arrow in the middle to copy it to my custom file. It's just that the arrow is quite small and occasionally I miss it, clicking the scroll bar instead.