TikZ: Text matching the steps of a grid and thus the height of a rectangle in TikZ?

Question

Let's assume I have a TikZ drawing:

• there are two rectangles of different heights
• next to each rectangle there is some text to be added (e.g. A and Z)
• the upper text has to be scalable in the way that it matches the steps of the diagram (here 3 steps or half)
• the lower text has to match the height of the rectangle it ends up next to exactly
• all of the text should be left-aligned at exactly the same grid line
• the whole drawing should be scalable (text and shapes)

My rather clumsy solution was to keep changing the font size until I've reached a result I deemed more or less satisfactory (you can see that it's still far from perfect). Is there any way to calculate this so that I can set a font size of n grid steps so that I can set A to be exactly 6 steps high, for instance?

This is how far I've come:

\documentclass{article}
\RequirePackage{lmodern}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[thick,scale=1, every node/.style={transform shape}]
\draw[step=1cm,gray,thin] (-1,-1) grid (10,10);
\fill[gray] (0,0) -- (4,0) -- (4,2) -- (0,2) -- (0,0);
\draw[red, ultra thick,|<->|] (5,0) -- (5,2);
\node[font={\fontsize{83}{0}\selectfont\bfseries}] at (7,1) {Z};
\fill[gray] (0,3) -- (4,3) -- (4,9) -- (0,9) -- (0,3);
\draw[green, ultra thick,|<->|] (5,3) -- (5,6);
\node[font={\fontsize{124}{0}\selectfont\bfseries}] at (7,4.45) {A};
\end{tikzpicture}
\end{document}

And this is how it looks like:

Update: Further research

Before everything else I'll have to give credit to AndréC for spreading the word and attracting other experts to post comments and answers – thank you!

I have received several very helpful comments which have given me the right key words to further my research.

Apparently, TeX does only know the size of a bounding box of a glyph, but not the size of the glyph itself. I have learned this from a commenter who's opinion I highly regard. I did my due diligence and tried to both confirm and refute her statement. Yet, the more I searched the more it was confirmed (here, here, and there) that there will always be some trial and error involved when it comes to the measurement and exact placement of glyphs/text inside the grid.

Answer 1

Finished.

import animate;
settings.tex="pdflatex"; 
settings.outformat="pdf";
animation Ani;
size(300);
void grid(pair A, pair B, pen p=currentpen)
{
for (int i=(int) A.x+1; i < B.x; ++i) { draw((i,A.y)--(i,B.y),p); }
for (int j=(int) A.y+1; j < B.y; ++j) { draw((A.x,j)--(B.x,j),p); }
draw(box(A,B),p);
}
grid((-1,-1),(10,10),gray);
fill((0,0) -- (4,0) -- (4,2) -- (0,2)--cycle,gray);
fill((0,3) -- (4,3) -- (4,9) -- (0,9)--cycle,gray);
for(int i=1; i<=10; ++i){
save();
real height=i/10;
path[] checkA=texpath("Asymptote");
int a=4,b=6;
pair minA=min(checkA),maxA=max(checkA);
real le=abs((maxA.x,minA.y)-minA),he=abs((minA.x,maxA.y)-minA);
transform sca=scale(heighta/le,heightb/he);
minA=min(scacheckA);
maxA=max(scacheckA);
pair cente=(minA+maxA)/2;
fill(shift((5,3)+(cente-minA))scacheckA);
path[] checkZ=texpath("My drawing tool");
int a=4,b=2;
pair minZ=min(checkZ),maxZ=max(checkZ);
real le=abs((maxZ.x,minZ.y)-minZ),he=abs((minZ.x,maxZ.y)-minZ);
transform sca=scale(heighta/le,heightb/he);
minZ=min(scacheckZ);
maxZ=max(scacheckZ);
pair cente=(minZ+maxZ)/2;
fill(shift((5,0)+(cente-minZ))scacheckZ);
Ani.add();
restore();
}
erase();
Ani.movie(BBox(3mm,Fill(white)));

Asymptote can actually do it!

You can check step by step to understand my code.

The function path[] texpath(Label L) returns the path array that TEX would fill to draw the Label L.

size(300);
void grid(pair A, pair B, pen p=currentpen)
{
for (int i=(int) A.x+1; i < B.x; ++i) { draw((i,A.y)--(i,B.y),p); }
for (int j=(int) A.y+1; j < B.y; ++j) { draw((A.x,j)--(B.x,j),p); }
draw(box(A,B),p);
}
grid((-1,-1),(10,10),gray);
fill((0,0) -- (4,0) -- (4,2) -- (0,2)--cycle,gray);
fill((0,3) -- (4,3) -- (4,9) -- (0,9)--cycle,gray);
path[] checkA=texpath("A");
int a=4,b=6;
pair minA=min(checkA),maxA=max(checkA);
real le=abs((maxA.x,minA.y)-minA),he=abs((minA.x,maxA.y)-minA);
transform sca=scale(a/le,b/he);
minA=min(scacheckA);
maxA=max(scacheckA);
pair cente=(minA+maxA)/2;
fill(shift((5,3)+(cente-minA))scacheckA);
path[] checkZ=texpath("Z");
int a=4,b=2;
pair minZ=min(checkZ),maxZ=max(checkZ);
real le=abs((maxZ.x,minZ.y)-minZ),he=abs((minZ.x,maxZ.y)-minZ);
transform sca=scale(a/le,b/he);
minZ=min(scacheckZ);
maxZ=max(scacheckZ);
pair cente=(minZ+maxZ)/2;
fill(shift((5,0)+(cente-minZ))scacheckZ);