I didn't try it, but check this link on the same question

http://www.microsoft.com/office/community/en-us/default.mspx?dg=microsoft.public
.access.forms&mid=dc301743-5684-4750-96d4-83514c64b9dc

You can't make it bigger but you can work around it.
Here is the coding needed.

Set your actual CheckBoxName.Visible to No.

Add an unbound label to the form detail section.
Set its Caption to " "  ( a space)
Set it's Font to WingDings
Set it's font size to whatever you want (perhaps 24)
Place this label where you wish to see the check mark.
Set it's special effects to Sunken (if you want).
Set it's BackColor to White (if you want).
I've named it LabelLargeCheck.

Code the new Label's Click event:

Private Sub LabelLargeCheck_Click()
[CheckBoxName] = Not ([CheckBoxName])
If [CheckBoxName] = True Then
   LabelLargeCheck.Caption = Chr(252)
Else
   LabelLargeCheck.Caption = " "    ' a space
End If
End Sub
==========
Code the Form Current Event:

Private Sub Form_Current()
If Me.CheckBoxName = True Then
   LabelLargeCheck.Caption = Chr(252)
Else
   LabelLargeCheck.Caption = " "   ' a space
End If
End Sub
===========

If you want a CheckBox field label then just add another unbound label
and set its caption to the CheckBox field name. Leave it Visible.

Don't forget to change the name of the CheckBoxName in this coding to
whatever your Checkbox field is.

After you open the form and use the check box label,
re-adjust the font size if needed, and the size of the label,
to square it around the check mark.
That should do it.

Note: because it's a label now, you can also change it's color, if you
want.

Clicking on the new Label is equivalent to clicking on the CheckBox
field itself.

Check my March, 2004 "Access Answers" column in Pinnacle Publication's
"Smart Access"

You can download the column (and sample database) for free at
http://www.accessmvp.com/djsteele/SmartAccess.html