I have a database on Size and Return for every day in a year. It looks like this:

Day Return Size
1 5% 100
2 3% 103
....
10 1% .
11 5% .
12 4% 110

My Size Variable has some missing values, let's say I have size on Jan 31st, Feb 29th, but I don't have the size for the days in between. I would like to interpolate the size for the rest of the days.