Summary
Your UFO will be detected, but:
Only by primary radars used by military. ATC centers usually use secondary radars, but they can also receive primary feeds in some cases. So ensure your ATC center has a primary radar source. In that case the size won't appear on ATC display, only a standard track, like for any aircraft.
ATC secondary radar detects only cooperative targets, i.e. targets with a system aboard (ATC transponder) replying to interrogations from the ground, or from other aircraft for collision avoidance.
Many tracking/detection systems in aviation work only with cooperative targets and don't see the other. Tracking sites like FR24 display only cooperative aircraft.
Military with their primary radars can see the actual size, but a target is visible on primary radars only if it reflects radar waves, it must be somehow conductive.
Stationary targets at low elevation over the horizon are difficult to extract from echos created by ground, buildings, etc. Most (primary) radars discard echos from stationary targets at low elevation.
Details follow. You can also create a chat room for additional discussion about details. This way anyone interested can freely provide information without having to meet the rules and format of a formal answer.
Primary vs. secondary radar
ATC radars are secondary surveillance radars (SSR). These radars actually interrogate a system, the ATC transponder, aboard the target. Without entering into details SSR work based with transponders of two compatible kinds: the older mode A/C and the modern mode S. If you need more information, we can provide it.
If the target isn't equipped with a transponder, or if the transponder is off, ATC SSR won't see anything. This is how MH370 disappeared from ATC radars soon after takeoff, while still in the coverage of land ATC centers. The transponder system became inoperative for some reason (unit switched off, or unit/antenna deteriorated).
However military have to detect uncooperative targets, and they use primary surveillance radars (PSR), the good old echo radar working by reflection on the target. In that case the radar station just sends a radio pulse and wait for any echo.
PSR stations may also be available at some ATC centers.
PSR stations use the Doppler shift to assess the target velocity. Stationary targets have no Doppler shift, the echo wave has the same frequency as the wave sent by the station. PSR stations may discard echos from stationary targets at low elevation, since most of them are just echos from buildings, trees, lakes, ground, etc. So your stationary object might be discarded and not visible.
Not all materials are able to reflect waves of a certain frequency. Usually this is limited to conductive materials which can react to the wave as if it was a transmitting antenna efficient for that frequency (technically it must be an electrical dipole). If your UFO is made of plastic with organic creatures inside, it will be permeable to waves and will reflect nothing.
SSR and PSR data can be mixed
In some countries, PSR and SSR stations are part of a network with a hub where all data from all stations are centralized, processed, correlated and redistributed to all ATC and military centers. In that case ATC operators will see an unidentified target detected by a PSR, without the exact altitude information (a PSR system is usually able to detect the direction, the velocity and the range).
PSR data may be discarded
Military are highly interested in uncooperative targets, ATC is not. It is possible ATC operators choose to not display PSR targets, only SSR targets. A reason is collision avoidance is based on altitude, and a target without an altitude cannot be managed. There are ATC operators on this stack, they may provide additional details about that.
ATC operator may not see the unidentified target. If they see it, the size won't be indicated, a target being basically a line segment (track) with SSR data (transponder id, altitude, speed, vertical speed) displayed as a label close to the track. As the operator who assigns the transponder id ("squawk code" in mode A/C) can associate this id with the aircraft flight plan and the flight number, often this flight number replaces the transponder id on ATC displays.
Whatever, if military detect an unidentified target, they will inform related civil ATC units. Actually it's possible a military controller will be present in the civil ATC unit, working with a civil controller to coordinate military and civil traffics.
Weather radar
Many civil aircraft have a weather radar onboard, they may detect the uncooperative target, as these radars work by reflection on water droplets (rain in the cloud and below them). These radars show somehow the actual horizontal shape of the target by reflecting on droplets at any distance. However a solid target may be impermeable to waves, echos will be limited to the envelope portion facing the aircraft, the target will appear as a silhouette.
Collision avoidance system
Many aircraft (incl. all commercial aircraft) have a collision avoidance system aboard. This system works like a SSR station, it interrogates the transponder of other aircraft in the vicinity, to know their position and altitude. Such systems won't see any target not equipped with a transponder.