ADo most do 4" or 6" smash cakes? I want to offer complimentary smash cakes with first birthday cakes but I'm not sure what size is best. I always did 6" cakes for my kids but it does leave a lot of wasted cake.

AWhat about a jumbo cupcake?

I would do 4" personally...as you said a lot gets wasted.

I offer little 4" in a color that matches the tiered cake, which is the only way I offer them. Free with a tiered cake order (1st bday only)

AI prefer 4". Anything bigger than that just looks silly next to a widdle baby imo.

All of our smash cakes here are 6" singles.   I give it free if the order is over $100.   If not, for $10 it will be simply decorated to match the main cake.   I have had some fancy schmancy 6" single smash cakes that ended up being $25 after all the bells 'n whistles were tallied up.   I have also done many 6" doubles for the child's professional 1st birthday photo. 

AI plan to do a smash cake in coordinating colors with either the child's name or the number one in buttercream.

I think I'm going to do a two layer 4" cake.

I do one layer 6 inch smash in buttercream only. 

I also do a single layer 6"cake that is simply decorated. If I have any left over decorations from the main cake I will add them to the smash cake.

I typically throw in a 4 inch two layer cake when they spend $75+. It has to be in the same flavor as the larger cake. Today, I had a mother question the 4 in, saying it was too small and that she wanted a six inch. Oh and that the two layer was too small and that it should be taller than 2 inches. She ended up increasing the size of the big cake so I threw in the 6 in cake. She was one of those customers that couldn't make up her mind so I pretty much agreed to what she eventually wanted. 

We just did my niece's 1st birthday party. I made a standard size (4 layer) 6 in cake because I wanted the cake for my portfolio. It was clearly too much cake for her! I tried to explain that to the customer today but she said she wanted it to look good in pictures. Oh boy. 

For 1st birthdays only, I do a complimentary jumbo cupcake in the same flavor as the main cake and, depending on the theme of the big cake, toss in a decoration or two to match. Most people just see the word "complimentary" and are very happy. I haven't yet been asked for a smash cake for an older birthday or for something bigger than the jumbo cupcake. If I was, I would charge accordingly. It's easy enough to pinch some batter from the main cake and toss it in a cupcake pan. If I have to make more batter for a 6" cake and then fill & frost it, they are paying :)

AI like 4" for a smash cake. Like others, I offer it for no charge with a first birthday cake. I don't advertise that though; I wait until all the details of the order are worked out and then I offer it. If they are a huge PITA already I don't bother!