I have a database field that I want to store various products sizes

So I have a form to edit this record with the 1 size field called 'size'

checkbox input choice seems right:

<input name="size1" type="checkbox" value="small" checked />
<input name="size2" type="checkbox" value="medium" checked />
<input name="sizes3" type="checkbox" value="large" checked />

- when I save - I can concatente them together - sep with comma like

size = size1. ',' . size2 . ',' . size3


- but when I re-edit the field I'll have to UNDO that and pull each out and put back into

<input name="size1" and <input name="size2" and 3 etc...

Is there an easier way to deal with storing multiple choices that will be stored in 1 field?