Each of the following designs has been tested both in throughput and dimensions. Each design can achieve full belt saturation in its Tier (Yellow, Red, Blue) thanks to the to three individual belts taking care of the output before being merged into a single one. The designs can achieve full saturation for Iron plate recipe, Copper plate recipe and Stone Bricks recipe (in the case of the last two belts of stone are need to be used to feed the smelter). The inner Input belt is always the fuel input (wood, coal, solid fuel, rocket fuel).
Every tier has appropriate cover tiles for its tech level.
Tiers are following and include two versions of the same design -- one with input and output belts being on the same side and one with the input and output belts on opposite sides of a smelter (for a total of 6 blueprints):
Early-Game/Yellow Tier: Stone Furnaces, Wooden power poles, Yellow belts, Basic Electric Inserters, Long Arm Electric Inserters. No cover tiles.
Mid-Game/Red Tier: Steel Furnaces, Medium and Large Steel Electric Power Poles, Red Belts, one Roboport, Basic Electric Inserters, Long Arm Electric Inserters. Stone brick covering.
Late-Game/Blue Tier: Electric Furnaces w/Prod. Modules III's, Power Substations and Large Steel Electric Power Poles, Beacons w/Speed Modules III's, two Ropoports, Concrete covering (both normal and hazard).
NOTE: If the Early and Mid-Game designs are placed in a horizontal or vertical position next to each other, that has a single column or row of tiles free between each smelter schematic, then it is possible to place there the Late-Game set-up with Electric furnaces with the Beacons once you switch the inner fuel line into a resource line and remove the outer resource belt line.
IMPORTANT NOTE: These set-ups work only for a non-marathon settings game. Only for recipe's from a pure vanilla game.
EDIT1: Changed the Tier 2 and Tier 3 Same-side schematics (specifically the input/output section) to further make it easier to switch from one to the other.
EDIT2: To be frank. I am not going to be adapting these blueprints to the the new UG Belt update from Friday, 5th of May. Because through the grapevine I heard that the changes to the UG belts maximum length are not yet permanent.
EDIT3: Incorrectly named smelter BP's. Corrected now.