Resize single character from certain font

Question

I use Kerkis Sans greek font as a secondary font in my documents, for titles, paragraphs etc. As I've been testing the font I noticed that the capital greek F (Φ) is much smaller that the rest of the letters. My attempt to resize it was this
documentclass[english,greek]{article}
usepackage[T1]{fontenc}
usepackage{babel}
usepackage{txfontsb,graphicx}

newcommand{kerkissans}[1]{{fontfamily{maksf}selectfont #1}}

begin{document}
ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨΩ
kerkissans{ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨΩ}
kerkissans{textbf{ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨΩ}}
kerkissans{textbf{resizebox{!}{6pt}{Φ}Φ}}
end{document}

The problems I face are

This issue appears only with Kerkis Sans font and only when the text is bold
My attempt fixes the size locally

Is there a way to resize the bold Φ globally?

egreg · Accepted Answer

The bug should be reported to the maintainer of Kerkis.
In the meantime, you can do as follows:
documentclass[english,greek]{article}
usepackage[T1]{fontenc}
usepackage{babel}
usepackage{txfontsb}
usepackage{graphicx}
usepackage{newunicodechar}
usepackage{xparse}

newcommand{kerkissans}[1]{{fontfamily{maksf}selectfont #1}}

newunicodechar{Φ}{FIXEDPHI}

ExplSyntaxOn
NewDocumentCommand{FIXEDPHI}{}
 {
  str_if_eq:eeTF { use:c { f@family } } { maksf }
   {
    str_if_in:enTF { use:c { f@series } } { b }
     { resizebox{!}{fontcharhtfont`A}{F} +}
     { F }
   }
   { F }
 }
prg_generate_conditional_variant:Nnn str_if_in:nn { e } { T, F, TF }
ExplSyntaxOff

begin{document}

ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨΩ

kerkissans{ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨΩ}

kerkissans{textit{ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨΩ}}

kerkissans{textbf{ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨΩ}}

kerkissans{textit{textbf{ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨΩ}}}

end{document}

I added a + in order to show that the fix is applied only when necessary, that is, when the font series is bold (also bold italic has the bug).
In the production version, remove the +:
NewDocumentCommand{FIXEDPHI}{}
 {
  str_if_eq:eeTF { use:c { f@family } } { maksf }
   {
    str_if_in:enTF { use:c { f@series } } { b }
     { resizebox{!}{fontcharhtfont`A}{F} }
     { F }
   }
   { F }
 }

Output with the + for debugging

Output with the production version

Resize single character from certain font

One Answer

Output with the `+` for debugging

Output with the production version

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