My screen has a spline groove that's about 1/4 inch. What size spline do I need?

1/4th inch would be .250 inches. I suggest that you pull a small piece out and take it with you to your local hardware store to compare. About 1/4 inch is not a close enough measurement. I had several screens (I think 6 screens) that were "about" 1/8 inch. I found that there were three different sizes in the mix even though they "looked" to be "about" the same.

do i need to downsize the spline to install a longer lasting aluminum screen?

Amazon sent me your q since I recently bought this product. Check your screen to see if it currently has spline. If so you need to go ahead and get replacement. You might could use the the old but chances are it will rip enemies extracting, not last much longer, look lousy vs the new screen or all of the above.

What size spline do I need for a 5mm slot?

.125"=.32cm. and that will work on US screen s.

What are the dimensions of the spline?

The latest version of this spline I received is .097" thick and .298" wide. This is flexible spline and works well in my 18 year old screen room which takes more narrow spline than what is currently being sold at Lowe's and Home Depot.