Getting objects within a certain distance from another object?

To find all of the parts within a radius of a point, iterate through workspace recursively:

function getparts(point,radius)
	local list={}
	local f;function f(x)
		for _,v in pairs(x:GetChildren())do
			f(v)
			if v:IsA("BasePart")and(v.Position-point).magnitude<=radius then
				list[#list+1]=v
			end
		end
	end
	f(workspace)
	return list
end

to select them in studio, game.Selection:Set(getparts(point,radius))

If you're worried about efficiency for constant scanning, here's an optimization:

local allparts={}
do
	local f;function f(x)
		for _,v in pairs(x:GetChildren())do
			f(v)
			if v:IsA("BasePart")and v.ClassName~="Terrain"then
				allparts[v]=true
			end
		end
	end
	f(workspace)
end
workspace.DescendantAdded:connect(function(x)
	if x:IsA("BasePart")then
		allparts[x]=true
	end
end)
workspace.DescendantRemoving:connect(function(x)
	if x:IsA("BasePart")then
		allparts[x]=false
end)
function getparts(point,radius)
	local list={}
	for v,_ in pairs(allparts)do
		if(v.Position-point).magnitude<=radius then
			list[#list+1]=v
		end
	end
	return list
end