What size cake pan could be substitued in place of muffins?
Which pan should I use?

Volume of a regular muffins
1/3 cup equals about 79 ml
1/4 cup equals about 60 ml
--> I take the average of both: 70ml per regular muffin

Volume of mini muffins
1/8 cup equals about 30 ml
2 tablespoons equal about 20 ml
--> I take the average of both: 25ml per mini muffin

Total volume 
24 mini muffins ⋅ 25 ml/mini muffin + 4 ⋅ 70 ml/regular muffin = 880 ml = 880 cm³ batter

Using the 8x8 pan 
8" ⋅ 8" = 20.32 cm ⋅ 20.32 cm ≈ 413 cm². Height of the unbaked cake: 880 cm³ / 413 cm² ≈ 2 cm ≈ 0,75".

Using the 9x13 pan 
9" ⋅ 13" = 22.86 cm ⋅ 33.02 cm ≈ 755 cm². Height of the unbaked cake: 800 cm³ / 755 cm² ≈ 1 cm ≈ 0,4 inches.

I assume that baked muffins will gain the same the volume as the volume of the unbaked batter. Or at least 3/4 of their original volume.




The cake without streusel topping in the 8" x 8" pan will be 4 cm or 1,5" thick and the cake in the 9" x 13" pan will be 2 cm or 0,8" thick on the average. After baking, the cake in the 8" x 8" pan might look like a molehill cake :D

After adding streusel topping, I assume you cake will be 1 cm thicker. To prevent extreme doming and to have a shorter baking time I recommend using the 9" x 13" pan. 

Baking time

Some receipes in my cook / baking book say that pound cakes backed directly on a sheet pan take about 25 to 30 minutes at 180°C / 356F (in an electric oven) to de done. You should still check your cake continuously after 15 minutes. 



image sources:
https://commons.wikimedia.org/wiki/File:Muffins_with_honey.jpg
https://commons.wikimedia.org/wiki/File:Muffin_Starbucks_2.jpg