To Do This Simply Follow These Instructions:
1.Go To Admincp
2.Styles & Templates
3.Search In Templates
4.Search ' editor_contents.css '
Here You Can Change Font , Background's Color And The Default Size On The New Editor.

Example Edited Code: (Tahoma Font , Light Blue Background's Color)

body
{
/* Font */
font-family: Tahoma;
font-size: 12px;

/* Text color */
color: #222;

/* Remove the background color to make it transparent */
background-color: #e8f5ff;
}

html
{
/* #3658: [IE6] Editor document has horizontal scrollbar on long lines
To prevent this misbehavior, we show the scrollbar always */
_overflow-y: scroll
}

img:-moz-broken
{
-moz-force-broken-image-icon : 1;
width : 24px;
height : 24px;
}
img, input, textarea
{
cursor: default;
}

img.previewthumb {
max-width:150px;
max-height:150px;
height:auto !important;
width:auto !important;
width:150px;
height:150px;
margin:1px;
}

img.size_thumbnail {
max-height:{vb:stylevar image_thumbnail_max};
max-width:{vb:stylevar image_thumbnail_max};
}

img.size_medium {
max-height:{vb:stylevar image_small_max};
max-width:{vb:stylevar image_small_max};
}

img.size_large {
max-height:{vb:stylevar image_large_max};
max-width:{vb:stylevar image_large_max};
}

img.size_fullsize {
max-width:100%;
max-height:100%;
overflow:hidden;
}

img.align_left {
float:left; /* dont use stylevar for this */
margin: 1em;
margin-left: 0;
}

img.align_center {
margin: 0 auto;
display:block;
}

img.align_right {
float:right; /* dont use stylevar for this */
margin: 1em;
margin-right: 0;
}

/* Table tag */
table {
width: auto;
}
table.wysiwyg_dashes {
border-collapse: collapse;
}

td.wysiwyg_dashes_td {
border: {vb:stylevar editor_wysiwyg_table_borderSize} dotted {vb:stylevar editor_wysiwyg_table_borderColor};
padding: 3px;
}

/* paragraph tag */
p {
margin: 0px;
padding: 0px;
}

/* preview break tag */
hr.previewbreak {
background-color:red !important;
border:medium none !important;
color:red !important;
height:6px !important;
}

/* pagebreak tag */
h3.wysiwyg_pagebreak {
border: 1px dashed #cccccc;
border-top: 3px double black;
padding: 4px;
}


Regards,