well, A3 hatch, A3 Sedan, Golf, Golf GTI are are having same MQB platform. They have almost same front and rear legroom except the boot. In the last 5 months I have almost test driven every other week to decide. A3 is nice car for young buyer with small family. ( kids/toddler). A3 sedan has almost exactly same space what an A3 hatchback is and for that matter almost same as what GTI has.
2015 A3 Sedan
[TABLE="class: tabledimensions, width: 750"]
<tbody>[TR]
[TD="class: bold orange"]Width[/TD]
[TD]70.7 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Length[/TD]
[TD]175.4 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Height[/TD]
[TD]55.7 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Wheelbase[/TD]
[TD]103.8 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Front Headroom[/TD]
[TD]36.5 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Rear Headroom[/TD]
[TD]36.10 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Front Shoulder Room[/TD]
[TD]54.80 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Rear Shoulder Room[/TD]
[TD]53.00 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Front Legroom[/TD]
[TD]41.2 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Rear Legroom[/TD]
[TD]35.1 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Ground Clearance[/TD]
[TD]N/R inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Curb Weight[/TD]
[TD]3362 lbs.[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Cargo Capacity (Std/Max)[/TD]
[TD]10.00/26.00 cu.ft.[/TD]
[/TR]
</tbody>[/TABLE]
2014 Civic US
[TABLE="class: tabledimensions, width: 750"]
<tbody>[TR]
[TD="class: bold orange"]Width[/TD]
[TD]69.0 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Length[/TD]
[TD]179.4 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Height[/TD]
[TD]56.5 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Wheelbase[/TD]
[TD]105.1 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Front Headroom[/TD]
[TD]37.9 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Rear Headroom[/TD]
[TD]36.20 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Front Shoulder Room[/TD]
[TD]56.60 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Rear Shoulder Room[/TD]
[TD]53.30 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Front Legroom[/TD]
[TD]42.0 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Rear Legroom[/TD]
[TD]36.2 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Ground Clearance[/TD]
[TD]N/R inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Curb Weight[/TD]
[TD]2921 lbs.[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Cargo Capacity (Std/Max)[/TD]
[TD]12.50 cu.ft.[/TD]
[/TR]
</tbody>[/TABLE]
CLA 250 US
[TABLE="class: tabledimensions, width: 750"]
<tbody>[TR]
[TD="class: bold orange"]Width[/TD]
[TD]70.0 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Length[/TD]
[TD]182.3 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Height[/TD]
[TD]56.6 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Wheelbase[/TD]
[TD]106.3 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Front Headroom[/TD]
[TD]38.2 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Rear Headroom[/TD]
[TD]35.40 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Front Shoulder Room[/TD]
[TD]56.00 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Rear Shoulder Room[/TD]
[TD]53.20 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Front Legroom[/TD]
[TD]40.2 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Rear Legroom[/TD]
[TD]27.1 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Ground Clearance[/TD]
[TD]N/R inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Curb Weight[/TD]
[TD]3264 lbs.[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Cargo Capacity (Std/Max)[/TD]
[TD]13.10 cu.ft.[/TD]
[/TR]
</tbody>[/TABLE]
2015 Audi A4
[TABLE="class: tabledimensions, width: 750"]
<tbody>[TR]
[TD="class: bold orange"]Width[/TD]
[TD]71.9 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Length[/TD]
[TD]185.1 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Height[/TD]
[TD]56.2 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Wheelbase[/TD]
[TD]110.6 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Front Headroom[/TD]
[TD]38.0 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Rear Headroom[/TD]
[TD]37.50 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Front Shoulder Room[/TD]
[TD]55.50 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Rear Shoulder Room[/TD]
[TD]54.30 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Front Legroom[/TD]
[TD]41.3 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Rear Legroom[/TD]
[TD]35.2 inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Ground Clearance[/TD]
[TD]N/R inches[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Curb Weight[/TD]
[TD]3516 lbs.[/TD]
[/TR]
[TR]
[TD="class: bold orange"]Cargo Capacity (Std/Max)[/TD]
[TD]12.40/34.00 cu.ft.
[/TD]
[/TR]
</tbody>[/TABLE]
MK7 GTI

Asof your question, the above figures will give you a decent idea. Compare A3 and Civic dimensions for a better idea.