This is an extendion of m_goldberg's answer.

Create the following fraction (Esc pi Esc ctrl-/2)

in your notebook and paste it into the text for the tick label. Then wrap the text in Style and make it size 18. When it shrinks in the grid you will approximately the same size for all of the tick labels.

cp = ContourPlot[x^2 + y^2, {x, 0, \[Pi]}, {y, -\[Pi], \[Pi]}, FrameTicks -> {{{{-\[Pi]/2, Style["\!\(\*FractionBox[\(-\[Pi]\), \(2\)]\)", 18]}, 0, {\[Pi]/2, Style["\!\(\*FractionBox[\(\[Pi]\), \(2\)]\)", 18]}}, None}, {None, None}}, FrameTicksStyle -> {Directive[14], Directive[14]}]; 

and then

Grid[{{cp, cp}, {cp, cp}}] 

This is a fair amount of work. If you are doing this a great deal you will probably need to automate it.

This is an extendion of m_goldberg's answer.

Create the following fraction (Esc pi Esc ctrl-/2)

in your notebook and paste it into the text for the tick label. Then wrap the text in Style and make it size 18. When it shrinks in the grid you will approximately the same size for all of the tick labels.

cp = ContourPlot[x^2 + y^2, {x, 0, π}, {y, -π, π}, FrameTicks -> {{{{-π/2, Style["\!\(\*FractionBox[\(-π\), \(2\)]\)", 18]}, 0, {π/2, Style["\!\(\*FractionBox[\(π\), \(2\)]\)", 18]}}, None}, {None, None}}, FrameTicksStyle -> {Directive[14], Directive[14]}]; 

and then

Grid[{{cp, cp}, {cp, cp}}] 

This is a fair amount of work. If you are doing this a great deal you will probably need to automate it.