I have a lot of experience with the U.I. and this will be probably be way to much work for what you want.
Increasing font size is not all that hard but realize that when you change one section, others will change with it.
Copy the font folder from the reference files or the Trinity install dir.
C:\Program Files (x86)\Stardock Games\Sins of a Solar Empire - Trinity\Font
There are 2 types of font files
.ttf is the actual fonts file
.font is the size and type used (.ttf)
Example .font
TXT
Font
Height 16
TypeFaceName "SinsAgencyBold"
Try bumping the height up to 18 or 22
Know that all the buttons are meant to have the original height, increasing will cause the text to no longer fit the graphics.
The fleet icons is a whole new ballgame. You will need to expand or blow up the icons and and change all the brush locations. Brush is the text (in the Windows folder) the tells the engine what texture sheet (Textures\RaceLogos.tga) to look at, where on the sheet and how big.
\Mods-Diplomacy v1.34\Reference Diplomacy\Window\NPC.playerThemes
\Mods-Diplomacy v1.34\Reference Diplomacy\Window\PlayerPhase.playerThemes
\Mods-Diplomacy v1.34\Reference Diplomacy\Window\PlayerPsi.playerThemes
\Mods-Diplomacy v1.34\Reference Diplomacy\Window\PlayerTech.playerThemes
\Mods-Diplomacy v1.34\Reference Diplomacy\Window\Special.playerThemes
\Mods-Diplomacy v1.34\Reference Diplomacy\Window\SpecialNPC.playerThemes
TXT
themeCount 2
Icon
textureName "RaceLogos.tga"
startTopLeft [1,911]
width 32
height 32
rowItemCount 14
horizontalStride 40
verticalStride 40
IconOverlay
textureName "RaceLogos.tga"
startTopLeft [1440 , 851]
width 14
height 14
rowItemCount 14
horizontalStride 20
verticalStride 20
Picture
textureName "RaceLogos.tga"
startTopLeft [1,330]
width 105
height 105
rowItemCount 14
horizontalStride 110
verticalStride 110
PlanetIcon
textureName "RaceLogos.tga"
startTopLeft [731 , 911]
width 23
height 23
rowItemCount 14
horizontalStride 40
verticalStride 40
Take a look at all the files involved and start a mod folder with all these files and let me know if you want to continue.
take a look at soase.x90x.net/?i=modding/UI-Buttons.html