Upvote:5
There is no standard size for under seat stowage because it varies greatly depending on the seat used and other equipment.
Yes, the curvature of the wall affects the dimensions. The placement of the IFE box affects the dimensions. Exit row proximity affects dimensions.
I'm reasonably sure American's IFE equipped 737's have the electronics under the middle seat but that affects mostly the height of the space. If the item is low profile, 4-5", I'd say the middle seat offers the widest space (no guarantees though).
Be aware, depending on when this flight is, the aircraft may be swapped due to normal schedule changes and the MAX grounding.