Forcing page number on a page with large figure

The number is overprinted over the image, but may not be visible depending on the colours:

probably the easiest way to move the footline is the geometry package:

documentclass{article}
usepackage{graphicx,geometry}
begin{document}

aaa

bbbb


begin{figure}[p]
centering
includegraphics[height=.9paperheight]{example-image-9x16}


end{figure}


clearpage

newgeometry{textheight=1.3textheight}

begin{figure}[p]
centering
includegraphics[height=.9paperheight]{example-image-9x16}


end{figure}

clearpage

restoregeometry

end{document}

Firstly, I'd suggest setting the float using

begin{figure}[p]
  % figure content here
end{figure}

This would allow the float to be placed within the text, yet float to a page on its own. Secondly, use fancyhdr to design a float-specific page style that sets the page number lower than usual. Finally, use floatpag to specify this new float page style for that specific float:

documentclass{article}

usepackage{graphicx,floatpag,fancyhdr}
usepackage{lipsum}

% This defines the fancy page style to be similar to plain
pagestyle{fancy}
fancyhf{}% Clear header/footer
fancyfoot[C]{thepage}
renewcommand{headrulewidth}{0pt}% Remove header rule

% Page style plainlower is similar to plain, but lowers page number by 6 lines of text
fancypagestyle{plainlower}{
  fancyhf{}% Clear header/footer
  fancyfoot[C]{raisebox{-6baselineskip}{thepage}}
  renewcommand{headrulewidth}{0pt}% Remove header rule
}

begin{document}

lipsum[1-2]

begin{figure}[p]
  thisfloatpagestyle{plainlower}
  centering
  includegraphics[height=1.1textheight]{example-image-9x16}
  caption{A very large figure}
end{figure}

lipsum[3-10]

end{document}

Related reference: Suppress page number for a single page that only contains one large table?

Vertically moving the page number up using the command vspace worked for me. E.g.

begin{figure}[p]
thisfloatpagestyle{plainlower}
  centering
  includegraphics[height=1.1textheight]{example-image-9x16}
  caption{A very large figure}
end{figure}

vspace*{-6ex}
fillandplacepagenumber

Note this uses the fillandplacepagenumber command from: Alexander Perlis's Answer