For a quick solution, you could place all your fields in a fixed size table. (Eg: <table border="0" width="600" height="400">)
Another solution involves javascript, and opening a new window that is sized to your specifications. I'm not sure if you can resize the default browser window when a client navigates to your site.
Javascript: In your body tag, put this
onload="javascript: window.open('urlofpage.htm','nameofpage','height=400,width=600')"
Additional properties to add:
toolbar=yes/no
menubar=yes/no
location=yes/no
scrollbars=1/0
resizable=yes/no
status=yes/no
border=yes/no
screenx=# Horz position in Netscape
screeny=# Vert position in Netscape
left=# Horz position in IE
top=# Vert position in IE