I had a 32 oz Mr Stripey last yr..so bigger than a softball. Most on the garden forums dont like them tho because they are not prolific. I love the sweet taste and have grown them for many yrs. Sometimes I get a plant that doesnt produce a single tomato. I generally have several plants so it doesnt matter to me.

The size would depend on which Mr. Stripey you have. There is one that was developed in the UK by Glasshouse Research (if I recall correctly) whose original or correct name is Tigerella. It is mostly red with yellow stripes and averages around 2-3 oz.

The other is a beefsteak tomato from the United States and is mostly yellow with red stripes and it is usually at least a pound, with some much bigger as bigpinks says.

I've linked to a Google image search below. In the first row, the first two tomatoes are the American Mr. Stripey, and the third one is Tigerella aka Mr. Stripey.

Here is a link that might be useful: Google Image search for Mr. Stripey

Mine were tomato sized last year. About the size of a woman's fist. But boy were they delicious ! ! ! They were planted late last year when I shovel pruned a rose and needed something in that space, so production wasn't great and they had to fight with the weeds. But I planted three this year and am looking forward to them more than any of the others !

Here is a link that might be useful: Some old tomatoes

Here is one of my Mr. Stripeys, he is just in front of the vase.

I grew one plant last year and got one tomato. It was a good tomato.
Despite the really poor yields I just picked up a plant for this year.