Column makes fonts smaller

There is an option for Column or Grid to avoid this: AllowScriptLevelChange

cell = Framed[Style[(2*x^(3/2))/3, FontSize -> 14], 
   FrameMargins -> 10, 
   FrameStyle -> {AbsoluteThickness[1], LightGray}, 
   RoundingRadius -> 3];
{cell, Column[{cell}, AllowScriptLevelChange -> False]}

UPDATE

Honestly, I don't know why Panel doesn't respect AllowScriptLevelChange. As a workaround you can use ScriptLevel -> 0 inside Style to fix the size of Panel with the title

cell2 = Framed[Style[(2*x^(3/2))/3, FontSize -> 14, ScriptLevel -> 0],
    FrameMargins -> 10, 
   FrameStyle -> {AbsoluteThickness[1], LightGray}, 
   RoundingRadius -> 3];

Panel[Column[{Item[cell]}], "Hello"]
Panel[Column[{Item[cell2]}], "Hello"]

Another approach is to set AllowScriptLevelChange -> False via GridBoxOptions:

{cell, Style[
  Panel[Column[{cell}], "Hello", DefaultBaseStyle -> None],
  GridBoxOptions -> {AllowScriptLevelChange -> False}]}

---

For curiosity, here is a low-level workaround based on AllowScriptLevelChange -> False:

{cell,
 RawBoxes@Replace[ToBoxes[
    Panel[Column[{cell}], "Hello", DefaultBaseStyle -> None]], 
   gb_GridBox :> 
    RuleCondition@Append[gb, AllowScriptLevelChange -> False], -1]}

---

Honestly, I don't know why Panel doesn't respect AllowScriptLevelChange.

Actually PanelBox doesn't change the ScriptLevel, but when labeled Panel is used, label is added by wrapping the contents of the panel by GridBox which doesn't inherit AllowScriptLevelChange -> False (what I would count as a bug) and hence changes the ScriptLevel. The same is true for Labeled and the same workaround works:

{cell, Labeled[cell, "Hello", Top],
 RawBoxes@Replace[ToBoxes[
    Labeled[cell, "Hello", Top]], 
   gb_GridBox :> RuleCondition@Append[gb, AllowScriptLevelChange -> False], -1]}